diff --git a/substrate/core/client/db/src/lib.rs b/substrate/core/client/db/src/lib.rs
index 8d3b30c13d..521781fc64 100644
--- a/substrate/core/client/db/src/lib.rs
+++ b/substrate/core/client/db/src/lib.rs
@@ -153,6 +153,7 @@ impl<Block: BlockT> BlockchainDb<Block> {
 		let mut meta = self.meta.write();
 		if number == Zero::zero() {
 			meta.genesis_hash = hash;
+			meta.finalized_hash = hash;
 		}
 
 		if is_best {
@@ -178,6 +179,7 @@ impl<Block: BlockT> client::blockchain::HeaderBackend<Block> for BlockchainDb<Bl
 			best_hash: meta.best_hash,
 			best_number: meta.best_number,
 			genesis_hash: meta.genesis_hash,
+			finalized_hash: meta.finalized_hash,
 		})
 	}
 
@@ -411,59 +413,42 @@ impl<Block: BlockT> Backend<Block> {
 	}
 
 	// write stuff to a transaction after a new block is finalized.
-	//
-	// this manages state pruning and ensuring reorgs don't occur.
-	// this function should only be called if the finalized block is contained
-	// in the best chain.
-	fn note_finalized(&self, transaction: &mut DBTransaction, f_header: &Block::Header, f_hash: Block::Hash) -> Result<(), client::error::Error> {
-		const NOTEWORTHY_FINALIZATION_GAP: u64 = 32;
-
-		// TODO: ensure this doesn't conflict with old finalized block.
+	// this manages state pruning. Fails if called with a block which
+	// was not a child of the last finalized block.
+	fn note_finalized(
+		&self,
+		transaction: &mut DBTransaction,
+		f_header: &Block::Header,
+		f_hash: Block::Hash,
+	) -> Result<(), client::error::Error> {
 		let meta = self.blockchain.meta.read();
 		let f_num = f_header.number().clone();
+		if &meta.finalized_hash != f_header.parent_hash() {
+			return Err(::client::error::ErrorKind::NonSequentialFinalization(
+				format!("Last finalized {:?} not parent of {:?}",
+					meta.finalized_hash, f_hash),
+			).into())
+		}
 		transaction.put(columns::META, meta_keys::FINALIZED_BLOCK, f_hash.as_ref());
 
-		let (last_finalized_hash, last_finalized_number)
-			= (meta.finalized_hash.clone(), meta.finalized_number);
-
-		let finalized_gap = f_num - last_finalized_number;
-
-		if finalized_gap.as_() >= NOTEWORTHY_FINALIZATION_GAP {
-			info!(target: "db", "Finalizing large run of blocks from {:?} to {:?}",
-				(&last_finalized_hash, last_finalized_number), (&f_hash, f_num));
-		} else {
-			debug!(target: "db", "Finalizing blocks from {:?} to {:?}",
-				(&last_finalized_hash, last_finalized_number), (&f_hash, f_num));
-		}
-
-		let mut canonicalize_state = |canonical_hash| {
-			let commit = self.storage.state_db.canonicalize_block(&canonical_hash);
-			apply_state_commit(transaction, commit);
-		};
-
-		// when finalizing a block, we must also implicitly finalize all the blocks
-		// in between the last finalized block and this one. That means canonicalizing
-		// all their states in order.
 		let number_u64 = f_num.as_();
 		if number_u64 > self.pruning_window {
 			let new_canonical = number_u64 - self.pruning_window;
-			let best_canonical = self.storage.state_db.best_canonical();
 
-			for uncanonicalized_number in (best_canonical..new_canonical).map(|x| x + 1) {
-				let hash = if uncanonicalized_number == number_u64 {
-					f_hash
-				} else {
-					read_id::<Block>(
-						&*self.blockchain.db,
-						columns::HASH_LOOKUP,
-						BlockId::Number(As::sa(uncanonicalized_number))
-					)?.expect("existence of block with number `new_canonical` \
-						implies existence of blocks with all nubmers before it; qed")
-				};
+			let hash = if new_canonical == number_u64 {
+				f_hash
+			} else {
+				read_id::<Block>(
+					&*self.blockchain.db,
+					columns::HASH_LOOKUP,
+					BlockId::Number(As::sa(new_canonical))
+				)?.expect("existence of block with number `new_canonical` \
+					implies existence of blocks with all nubmers before it; qed")
+			};
 
-				trace!(target: "db", "Canonicalize block #{} ({:?})", uncanonicalized_number, hash);
-				canonicalize_state(hash);
-			}
+			trace!(target: "db", "Canonicalize block #{} ({:?})", new_canonical, hash);
+			let commit = self.storage.state_db.canonicalize_block(&hash);
+			apply_state_commit(transaction, commit);
 		};
 
 		Ok(())
@@ -507,7 +492,9 @@ impl<Block> client::backend::Backend<Block, Blake2Hasher, RlpCodec> for Backend<
 		})
 	}
 
-	fn commit_operation(&self, mut operation: Self::BlockImportOperation) -> Result<(), client::error::Error> {
+	fn commit_operation(&self, mut operation: Self::BlockImportOperation)
+		-> Result<(), client::error::Error>
+	{
 		let mut transaction = DBTransaction::new();
 		if let Some(pending_block) = operation.pending_block {
 			let hash = pending_block.header.hash();
@@ -527,19 +514,19 @@ impl<Block> client::backend::Backend<Block, Blake2Hasher, RlpCodec> for Backend<
 				// cannot find tree route with empty DB.
 				if meta.best_hash != Default::default() {
 					let parent_hash = *pending_block.header.parent_hash();
-					let tree_route = ::utils::tree_route::<Block>(
-						&*self.blockchain.db,
-						columns::HEADER,
-						meta.best_hash,
-						parent_hash,
+					let tree_route = ::client::blockchain::tree_route(
+						&self.blockchain,
+						BlockId::Hash(meta.best_hash),
+						BlockId::Hash(parent_hash),
 					)?;
 
 					// update block number to hash lookup entries.
 					for retracted in tree_route.retracted() {
 						if retracted.hash == meta.finalized_hash {
-							// TODO: can we recover here?
-							warn!("Safety failure: reverting finalized block {:?}",
+							warn!("Potential safety failure: reverting finalized block {:?}",
 								(&retracted.number, &retracted.hash));
+
+							return Err(::client::error::ErrorKind::NotInFinalizedChain.into());
 						}
 
 						transaction.delete(
@@ -699,6 +686,60 @@ mod tests {
 
 	type Block = RawBlock<u64>;
 
+	fn prepare_changes(changes: Vec<(Vec<u8>, Vec<u8>)>) -> (H256, MemoryDB<Blake2Hasher>) {
+		let mut changes_root = H256::default();
+		let mut changes_trie_update = MemoryDB::<Blake2Hasher>::new();
+		{
+			let mut trie = TrieDBMut::<Blake2Hasher, RlpCodec>::new(
+				&mut changes_trie_update,
+				&mut changes_root
+			);
+			for (key, value) in changes {
+				trie.insert(&key, &value).unwrap();
+			}
+		}
+
+		(changes_root, changes_trie_update)
+	}
+
+	fn insert_header(
+		backend: &Backend<Block>,
+		number: u64,
+		parent_hash: H256,
+		changes: Vec<(Vec<u8>, Vec<u8>)>,
+		extrinsics_root: H256,
+	) -> H256 {
+		use runtime_primitives::generic::DigestItem;
+		use runtime_primitives::testing::Digest;
+
+		let (changes_root, changes_trie_update) = prepare_changes(changes);
+		let digest = Digest {
+			logs: vec![
+				DigestItem::ChangesTrieRoot(changes_root),
+			],
+		};
+		let header = Header {
+			number,
+			parent_hash,
+			state_root: Default::default(),
+			digest,
+			extrinsics_root,
+		};
+		let header_hash = header.hash();
+
+		let block_id = if number == 0 {
+			BlockId::Hash(Default::default())
+		} else {
+			BlockId::Number(number - 1)
+		};
+		let mut op = backend.begin_operation(block_id).unwrap();
+		op.set_block_data(header, None, None, NewBlockState::Best).unwrap();
+		op.update_changes_trie(changes_trie_update).unwrap();
+		backend.commit_operation(op).unwrap();
+
+		header_hash
+	}
+
 	#[test]
 	fn block_hash_inserted_correctly() {
 		let db = Backend::<Block>::new_test(1);
@@ -924,6 +965,7 @@ mod tests {
 			assert!(backend.storage.db.get(::columns::STATE, &key.0[..]).unwrap().is_some());
 		}
 
+		backend.finalize_block(BlockId::Number(1)).unwrap();
 		backend.finalize_block(BlockId::Number(2)).unwrap();
 		assert!(backend.storage.db.get(::columns::STATE, &key.0[..]).unwrap().is_none());
 	}
@@ -932,54 +974,6 @@ mod tests {
 	fn changes_trie_storage_works() {
 		let backend = Backend::<Block>::new_test(1000);
 
-		let prepare_changes = |changes: Vec<(Vec<u8>, Vec<u8>)>| {
-			let mut changes_root = H256::default();
-			let mut changes_trie_update = MemoryDB::<Blake2Hasher>::new();
-			{
-				let mut trie = TrieDBMut::<Blake2Hasher, RlpCodec>::new(
-					&mut changes_trie_update,
-					&mut changes_root
-				);
-				for (key, value) in changes {
-					trie.insert(&key, &value).unwrap();
-				}
-			}
-
-			(changes_root, changes_trie_update)
-		};
-
-		let insert_header = |number: u64, parent_hash: H256, changes: Vec<(Vec<u8>, Vec<u8>)>| {
-			use runtime_primitives::generic::DigestItem;
-			use runtime_primitives::testing::Digest;
-
-			let (changes_root, changes_trie_update) = prepare_changes(changes);
-			let digest = Digest {
-				logs: vec![
-					DigestItem::ChangesTrieRoot(changes_root),
-				],
-			};
-			let header = Header {
-				number,
-				parent_hash,
-				state_root: Default::default(),
-				digest,
-				extrinsics_root: Default::default(),
-			};
-			let header_hash = header.hash();
-
-			let block_id = if number == 0 {
-				BlockId::Hash(Default::default())
-			} else {
-				BlockId::Number(number - 1)
-			};
-			let mut op = backend.begin_operation(block_id).unwrap();
-			op.set_block_data(header, None, None, NewBlockState::Best).unwrap();
-			op.update_changes_trie(changes_trie_update).unwrap();
-			backend.commit_operation(op).unwrap();
-
-			header_hash
-		};
-
 		let check_changes = |backend: &Backend<Block>, block: u64, changes: Vec<(Vec<u8>, Vec<u8>)>| {
 			let (changes_root, mut changes_trie_update) = prepare_changes(changes);
 			assert_eq!(backend.tries_change_storage.root(block), Ok(Some(changes_root)));
@@ -996,13 +990,76 @@ mod tests {
 		];
 		let changes2 = vec![(b"key_at_2".to_vec(), b"val_at_2".to_vec())];
 
-		let block0 = insert_header(0, Default::default(), changes0.clone());
-		let block1 = insert_header(1, block0, changes1.clone());
-		let _ = insert_header(2, block1, changes2.clone());
+		let block0 = insert_header(&backend, 0, Default::default(), changes0.clone(), Default::default());
+		let block1 = insert_header(&backend, 1, block0, changes1.clone(), Default::default());
+		let _ = insert_header(&backend, 2, block1, changes2.clone(), Default::default());
 
 		// check that the storage contains tries for all blocks
 		check_changes(&backend, 0, changes0);
 		check_changes(&backend, 1, changes1);
 		check_changes(&backend, 2, changes2);
 	}
+
+	#[test]
+	fn tree_route_works() {
+		let backend = Backend::<Block>::new_test(1000);
+		let block0 = insert_header(&backend, 0, Default::default(), Vec::new(), Default::default());
+
+		// fork from genesis: 3 prong.
+		let a1 = insert_header(&backend, 1, block0, Vec::new(), Default::default());
+		let a2 = insert_header(&backend, 2, a1, Vec::new(), Default::default());
+		let a3 = insert_header(&backend, 3, a2, Vec::new(), Default::default());
+
+		// fork from genesis: 2 prong.
+		let b1 = insert_header(&backend, 1, block0, Vec::new(), H256::from([1; 32]));
+		let b2 = insert_header(&backend, 2, b1, Vec::new(), Default::default());
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				backend.blockchain(),
+				BlockId::Hash(a3),
+				BlockId::Hash(b2)
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, block0);
+			assert_eq!(tree_route.retracted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![a3, a2, a1]);
+			assert_eq!(tree_route.enacted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![b1, b2]);
+		}
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				backend.blockchain(),
+				BlockId::Hash(a1),
+				BlockId::Hash(a3),
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, a1);
+			assert!(tree_route.retracted().is_empty());
+			assert_eq!(tree_route.enacted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![a2, a3]);
+		}
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				backend.blockchain(),
+				BlockId::Hash(a3),
+				BlockId::Hash(a1),
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, a1);
+			assert_eq!(tree_route.retracted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![a3, a2]);
+			assert!(tree_route.enacted().is_empty());
+		}
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				backend.blockchain(),
+				BlockId::Hash(a2),
+				BlockId::Hash(a2),
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, a2);
+			assert!(tree_route.retracted().is_empty());
+			assert!(tree_route.enacted().is_empty());
+		}
+	}
 }
diff --git a/substrate/core/client/db/src/light.rs b/substrate/core/client/db/src/light.rs
index 7d9d298833..cac32b8e6e 100644
--- a/substrate/core/client/db/src/light.rs
+++ b/substrate/core/client/db/src/light.rs
@@ -118,20 +118,18 @@ impl<Block> LightStorage<Block>
 		is_finalized: bool,
 	) {
 		let mut meta = self.meta.write();
-		if is_best {
-			if number == <<Block as BlockT>::Header as HeaderT>::Number::zero() {
-				meta.genesis_hash = hash;
-			}
 
+		if number == Zero::zero() {
+			meta.genesis_hash = hash;
+			meta.finalized_hash = hash;
+		}
+
+		if is_best {
 			meta.best_number = number;
 			meta.best_hash = hash;
 		}
 
 		if is_finalized {
-			if number == <<Block as BlockT>::Header as HeaderT>::Number::zero() {
-				meta.genesis_hash = hash;
-			}
-
 			meta.finalized_number = number;
 			meta.finalized_hash = hash;
 		}
@@ -152,6 +150,7 @@ impl<Block> BlockchainHeaderBackend<Block> for LightStorage<Block>
 			best_hash: meta.best_hash,
 			best_number: meta.best_number,
 			genesis_hash: meta.genesis_hash,
+			finalized_hash: meta.finalized_hash,
 		})
 	}
 
@@ -184,75 +183,43 @@ impl<Block> BlockchainHeaderBackend<Block> for LightStorage<Block>
 }
 
 impl<Block: BlockT> LightStorage<Block> {
-	// note that a block is finalized. ensure that best chain contains the finalized
-	// block number first.
+	// note that a block is finalized. only call with child of last finalized block.
 	fn note_finalized(&self, transaction: &mut DBTransaction, header: &Block::Header, hash: Block::Hash) -> ClientResult<()> {
-		const NOTEWORTHY_FINALIZATION_GAP: u64 = 32;
-
-		// TODO: ensure this doesn't conflict with old finalized block.
 		let meta = self.meta.read();
-		let f_num = header.number().clone();
-		let number_u64: u64 = f_num.as_();
-		transaction.put(columns::META, meta_keys::FINALIZED_BLOCK, hash.as_ref());
-
-		let (last_finalized_hash, last_finalized_number)
-			= (meta.finalized_hash.clone(), meta.finalized_number);
-
-		let finalized_gap = f_num - last_finalized_number;
-
-		if finalized_gap.as_() >= NOTEWORTHY_FINALIZATION_GAP {
-			info!(target: "db", "Finalizing large run of blocks from {:?} to {:?}",
-				(&last_finalized_hash, last_finalized_number), (&hash, f_num));
-		} else {
-			debug!(target: "db", "Finalizing blocks from {:?} to {:?}",
-				(&last_finalized_hash, last_finalized_number), (&hash, f_num));
+		if &meta.finalized_hash != header.parent_hash() {
+			return Err(::client::error::ErrorKind::NonSequentialFinalization(
+				format!("Last finalized {:?} not parent of {:?}",
+					meta.finalized_hash, hash),
+			).into())
 		}
 
+		transaction.put(columns::META, meta_keys::FINALIZED_BLOCK, hash.as_ref());
+
 		// build new CHT if required
-		let mut build_cht = |header: &Block::Header| -> ClientResult<()> {
-			if let Some(new_cht_number) = cht::is_build_required(cht::SIZE, *header.number()) {
-				let new_cht_start: NumberFor<Block> = cht::start_number(cht::SIZE, new_cht_number);
-				let new_cht_root = cht::compute_root::<Block::Header, Blake2Hasher, _>(
-					cht::SIZE, new_cht_number, (new_cht_start.as_()..)
-					.map(|num| self.hash(As::sa(num)).unwrap_or_default())
-				);
+		if let Some(new_cht_number) = cht::is_build_required(cht::SIZE, *header.number()) {
+			let new_cht_start: NumberFor<Block> = cht::start_number(cht::SIZE, new_cht_number);
+			let new_cht_root = cht::compute_root::<Block::Header, Blake2Hasher, _>(
+				cht::SIZE, new_cht_number, (new_cht_start.as_()..)
+				.map(|num| self.hash(As::sa(num)).unwrap_or_default())
+			);
 
-				if let Some(new_cht_root) = new_cht_root {
-					transaction.put(columns::CHT, &number_to_lookup_key(new_cht_start), new_cht_root.as_ref());
+			if let Some(new_cht_root) = new_cht_root {
+				transaction.put(columns::CHT, &number_to_lookup_key(new_cht_start), new_cht_root.as_ref());
 
-					let mut prune_block = new_cht_start;
-					let new_cht_end = cht::end_number(cht::SIZE, new_cht_number);
-					trace!(target: "db", "Replacing blocks [{}..{}] with CHT#{}", new_cht_start, new_cht_end, new_cht_number);
+				let mut prune_block = new_cht_start;
+				let new_cht_end = cht::end_number(cht::SIZE, new_cht_number);
+				trace!(target: "db", "Replacing blocks [{}..{}] with CHT#{}", new_cht_start, new_cht_end, new_cht_number);
 
-					while prune_block <= new_cht_end {
-						let id = read_id::<Block>(&*self.db, columns::HASH_LOOKUP, BlockId::Number(prune_block))?;
-						if let Some(hash) = id {
-							let lookup_key = number_to_lookup_key(prune_block);
-							transaction.delete(columns::HASH_LOOKUP, &lookup_key);
-							transaction.delete(columns::HEADER, hash.as_ref());
-						}
-						prune_block += <<Block as BlockT>::Header as HeaderT>::Number::one();
+				while prune_block <= new_cht_end {
+					let id = read_id::<Block>(&*self.db, columns::HASH_LOOKUP, BlockId::Number(prune_block))?;
+					if let Some(hash) = id {
+						let lookup_key = number_to_lookup_key(prune_block);
+						transaction.delete(columns::HASH_LOOKUP, &lookup_key);
+						transaction.delete(columns::HEADER, hash.as_ref());
 					}
+					prune_block += <<Block as BlockT>::Header as HeaderT>::Number::one();
 				}
 			}
-
-			Ok(())
-		};
-
-		// attempt to build CHT for all newly finalized blocks.
-		let last_finalized_u64 = last_finalized_number.as_();
-		for num in (last_finalized_u64..number_u64).map(|x| x + 1) {
-			let num = As::sa(num);
-			if num == f_num {
-				build_cht(header)?;
-			} else {
-				let old_header = match self.header(BlockId::Number(num))? {
-					Some(x) => x,
-					None => panic!("finalizing block {} implies existence of block {}; qed", f_num, num),
-				};
-
-				build_cht(&old_header)?;
-			}
 		}
 
 		Ok(())
@@ -260,8 +227,7 @@ impl<Block: BlockT> LightStorage<Block> {
 }
 
 impl<Block> LightBlockchainStorage<Block> for LightStorage<Block>
-	where
-		Block: BlockT,
+	where Block: BlockT,
 {
 	fn import_header(
 		&self,
@@ -285,11 +251,10 @@ impl<Block> LightBlockchainStorage<Block> for LightStorage<Block>
 				let meta = self.meta.read();
 				if meta.best_hash != Default::default() {
 					let parent_hash = *header.parent_hash();
-					let tree_route = ::utils::tree_route::<Block>(
-						&*self.db,
-						columns::HEADER,
-						meta.best_hash,
-						parent_hash,
+					let tree_route = ::client::blockchain::tree_route(
+						self,
+						BlockId::Hash(meta.best_hash),
+						BlockId::Hash(parent_hash),
 					)?;
 
 					// update block number to hash lookup entries.
@@ -377,6 +342,26 @@ pub(crate) mod tests {
 
 	type Block = RawBlock<u32>;
 
+	pub fn insert_block_with_extrinsics_root(
+		db: &LightStorage<Block>,
+		parent: &Hash,
+		number: u64,
+		authorities: Option<Vec<AuthorityId>>,
+		extrinsics_root: Hash,
+	) -> Hash {
+		let header = Header {
+			number: number.into(),
+			parent_hash: *parent,
+			state_root: Default::default(),
+			digest: Default::default(),
+			extrinsics_root,
+		};
+
+		let hash = header.hash();
+		db.import_header(header, authorities, NewBlockState::Best).unwrap();
+		hash
+	}
+
 	pub fn insert_block(
 		db: &LightStorage<Block>,
 		parent: &Hash,
@@ -486,6 +471,9 @@ pub(crate) mod tests {
 		assert_eq!(db.db.iter(columns::CHT).count(), 0);
 
 		// now finalize the block.
+		for i in (0..(cht::SIZE + cht::SIZE)).map(|i| i + 1) {
+			db.finalize_header(BlockId::Number(i)).unwrap();
+		}
 		db.finalize_header(BlockId::Hash(prev_hash)).unwrap();
 		assert_eq!(db.db.iter(columns::HEADER).count(), (1 + cht::SIZE + 1) as usize);
 		assert_eq!(db.db.iter(columns::CHT).count(), 1);
@@ -507,16 +495,79 @@ pub(crate) mod tests {
 		let db = LightStorage::new_test();
 
 		// insert 1 + SIZE + SIZE + 1 blocks so that CHT#0 is created
-		let mut prev_hash = Default::default();
-		for i in 0..1 + cht::SIZE + cht::SIZE + 1 {
+		let mut prev_hash = insert_block(&db, &Default::default(), 0, None);
+		for i in 1..1 + cht::SIZE + cht::SIZE + 1 {
 			prev_hash = insert_block(&db, &prev_hash, i as u64, None);
+			db.finalize_header(BlockId::Hash(prev_hash)).unwrap();
 		}
 
-		db.finalize_header(BlockId::Hash(prev_hash)).unwrap();
 		let cht_root_1 = db.cht_root(cht::SIZE, cht::start_number(cht::SIZE, 0)).unwrap();
 		let cht_root_2 = db.cht_root(cht::SIZE, (cht::start_number(cht::SIZE, 0) + cht::SIZE / 2) as u64).unwrap();
 		let cht_root_3 = db.cht_root(cht::SIZE, cht::end_number(cht::SIZE, 0)).unwrap();
 		assert_eq!(cht_root_1, cht_root_2);
 		assert_eq!(cht_root_2, cht_root_3);
 	}
+
+	#[test]
+	fn tree_route_works() {
+		let db = LightStorage::new_test();
+		let block0 = insert_block(&db, &Default::default(), 0, None);
+
+		// fork from genesis: 3 prong.
+		let a1 = insert_block(&db, &block0, 1, None);
+		let a2 = insert_block(&db, &a1, 2, None);
+		let a3 = insert_block(&db, &a2, 3, None);
+
+		// fork from genesis: 2 prong.
+		let b1 = insert_block_with_extrinsics_root(&db, &block0, 1, None, Hash::from([1; 32]));
+		let b2 = insert_block(&db, &b1, 2, None);
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				&db,
+				BlockId::Hash(a3),
+				BlockId::Hash(b2)
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, block0);
+			assert_eq!(tree_route.retracted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![a3, a2, a1]);
+			assert_eq!(tree_route.enacted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![b1, b2]);
+		}
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				&db,
+				BlockId::Hash(a1),
+				BlockId::Hash(a3),
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, a1);
+			assert!(tree_route.retracted().is_empty());
+			assert_eq!(tree_route.enacted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![a2, a3]);
+		}
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				&db,
+				BlockId::Hash(a3),
+				BlockId::Hash(a1),
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, a1);
+			assert_eq!(tree_route.retracted().iter().map(|r| r.hash).collect::<Vec<_>>(), vec![a3, a2]);
+			assert!(tree_route.enacted().is_empty());
+		}
+
+		{
+			let tree_route = ::client::blockchain::tree_route(
+				&db,
+				BlockId::Hash(a2),
+				BlockId::Hash(a2),
+			).unwrap();
+
+			assert_eq!(tree_route.common_block().hash, a2);
+			assert!(tree_route.retracted().is_empty());
+			assert!(tree_route.enacted().is_empty());
+		}
+	}
 }
diff --git a/substrate/core/client/src/backend.rs b/substrate/core/client/src/backend.rs
index 0a55a44949..aec4572b6c 100644
--- a/substrate/core/client/src/backend.rs
+++ b/substrate/core/client/src/backend.rs
@@ -108,11 +108,8 @@ where
 	fn begin_operation(&self, block: BlockId<Block>) -> error::Result<Self::BlockImportOperation>;
 	/// Commit block insertion.
 	fn commit_operation(&self, transaction: Self::BlockImportOperation) -> error::Result<()>;
-	/// Finalize block with given Id. This should also implicitly finalize all ancestors.
-	///
-	/// If the finalized block is not an ancestor of the current "best block", then
-	/// the chain will be implicitly reorganized to the best chain containing the newly
-	/// finalized block.
+	/// Finalize block with given Id. This should only be called if the parent of the given
+	/// block has been finalized.
 	fn finalize_block(&self, block: BlockId<Block>) -> error::Result<()>;
 	/// Returns reference to blockchain backend.
 	fn blockchain(&self) -> &Self::Blockchain;
diff --git a/substrate/core/client/src/blockchain.rs b/substrate/core/client/src/blockchain.rs
index d1af15a586..7a9904f8a0 100644
--- a/substrate/core/client/src/blockchain.rs
+++ b/substrate/core/client/src/blockchain.rs
@@ -77,11 +77,13 @@ pub enum ImportResult<E> {
 #[derive(Debug)]
 pub struct Info<Block: BlockT> {
 	/// Best block hash.
-	pub best_hash: <<Block as BlockT>::Header as HeaderT>::Hash,
+	pub best_hash: Block::Hash,
 	/// Best block number.
 	pub best_number: <<Block as BlockT>::Header as HeaderT>::Number,
 	/// Genesis block hash.
-	pub genesis_hash: <<Block as BlockT>::Header as HeaderT>::Hash,
+	pub genesis_hash: Block::Hash,
+	/// The head of the finalized chain.
+	pub finalized_hash: Block::Hash,
 }
 
 /// Block status.
@@ -92,3 +94,129 @@ pub enum BlockStatus {
 	/// Not in the queue or the blockchain.
 	Unknown,
 }
+
+/// An entry in a tree route.
+#[derive(Debug)]
+pub struct RouteEntry<Block: BlockT> {
+	/// The number of the block.
+	pub number: <Block::Header as HeaderT>::Number,
+	/// The hash of the block.
+	pub hash: Block::Hash,
+}
+
+/// A tree-route from one block to another in the chain.
+///
+/// All blocks prior to the pivot in the deque is the reverse-order unique ancestry
+/// of the first block, the block at the pivot index is the common ancestor,
+/// and all blocks after the pivot is the ancestry of the second block, in
+/// order.
+///
+/// The ancestry sets will include the given blocks, and thus the tree-route is
+/// never empty.
+///
+/// ```ignore
+/// Tree route from R1 to E2. Retracted is [R1, R2, R3], Common is C, enacted [E1, E2]
+///   <- R3 <- R2 <- R1
+///  /
+/// C
+///  \-> E1 -> E2
+/// ```
+///
+/// ```ignore
+/// Tree route from C to E2. Retracted empty. Common is C, enacted [E1, E2]
+/// C -> E1 -> E2
+/// ```
+#[derive(Debug)]
+pub struct TreeRoute<Block: BlockT> {
+	route: Vec<RouteEntry<Block>>,
+	pivot: usize,
+}
+
+impl<Block: BlockT> TreeRoute<Block> {
+	/// Get a slice of all retracted blocks in reverse order (towards common ancestor)
+	pub fn retracted(&self) -> &[RouteEntry<Block>] {
+		&self.route[..self.pivot]
+	}
+
+	/// Get the common ancestor block. This might be one of the two blocks of the
+	/// route.
+	pub fn common_block(&self) -> &RouteEntry<Block> {
+		self.route.get(self.pivot).expect("tree-routes are computed between blocks; \
+			which are included in the route; \
+			thus it is never empty; qed")
+	}
+
+	/// Get a slice of enacted blocks (descendents of the common ancestor)
+	pub fn enacted(&self) -> &[RouteEntry<Block>] {
+		&self.route[self.pivot + 1 ..]
+	}
+}
+
+/// Compute a tree-route between two blocks. See tree-route docs for more details.
+pub fn tree_route<Block: BlockT, Backend: HeaderBackend<Block>>(
+	backend: &Backend,
+	from: BlockId<Block>,
+	to: BlockId<Block>,
+) -> Result<TreeRoute<Block>> {
+	use runtime_primitives::traits::Header;
+
+	let load_header = |id: BlockId<Block>| {
+		match backend.header(id) {
+			Ok(Some(hdr)) => Ok(hdr),
+			Ok(None) => Err(ErrorKind::UnknownBlock(format!("Unknown block {:?}", id)).into()),
+			Err(e) => Err(e),
+		}
+	};
+
+	let mut from = load_header(from)?;
+	let mut to = load_header(to)?;
+
+	let mut from_branch = Vec::new();
+	let mut to_branch = Vec::new();
+
+	while to.number() > from.number() {
+		to_branch.push(RouteEntry {
+			number: to.number().clone(),
+			hash: to.hash(),
+		});
+
+		to = load_header(BlockId::Hash(*to.parent_hash()))?;
+	}
+
+	while from.number() > to.number() {
+		from_branch.push(RouteEntry {
+			number: from.number().clone(),
+			hash: from.hash(),
+		});
+		from = load_header(BlockId::Hash(*from.parent_hash()))?;
+	}
+
+	// numbers are equal now. walk backwards until the block is the same
+
+	while to != from {
+		to_branch.push(RouteEntry {
+			number: to.number().clone(),
+			hash: to.hash(),
+		});
+		to = load_header(BlockId::Hash(*to.parent_hash()))?;
+
+		from_branch.push(RouteEntry {
+			number: from.number().clone(),
+			hash: from.hash(),
+		});
+		from = load_header(BlockId::Hash(*from.parent_hash()))?;
+	}
+
+	// add the pivot block. and append the reversed to-branch (note that it's reverse order originalls)
+	let pivot = from_branch.len();
+	from_branch.push(RouteEntry {
+		number: to.number().clone(),
+		hash: to.hash(),
+	});
+	from_branch.extend(to_branch.into_iter().rev());
+
+	Ok(TreeRoute {
+		route: from_branch,
+		pivot,
+	})
+}
diff --git a/substrate/core/client/src/client.rs b/substrate/core/client/src/client.rs
index a2128aad93..43d04c2b72 100644
--- a/substrate/core/client/src/client.rs
+++ b/substrate/core/client/src/client.rs
@@ -21,7 +21,7 @@ use futures::sync::mpsc;
 use parking_lot::{Mutex, RwLock};
 use primitives::AuthorityId;
 use runtime_primitives::{bft::Justification, generic::{BlockId, SignedBlock, Block as RuntimeBlock}};
-use runtime_primitives::traits::{Block as BlockT, Header as HeaderT, Zero, One, As, NumberFor, CurrentHeight, BlockNumberToHash};
+use runtime_primitives::traits::{Block as BlockT, Header as HeaderT, Zero, As, NumberFor, CurrentHeight, BlockNumberToHash};
 use runtime_primitives::BuildStorage;
 use primitives::{Blake2Hasher, RlpCodec, H256};
 use primitives::storage::{StorageKey, StorageData};
@@ -40,7 +40,10 @@ use notifications::{StorageNotifications, StorageEventStream};
 use {cht, error, in_mem, block_builder, bft, genesis};
 
 /// Type that implements `futures::Stream` of block import events.
-pub type BlockchainEventStream<Block> = mpsc::UnboundedReceiver<BlockImportNotification<Block>>;
+pub type ImportNotifications<Block> = mpsc::UnboundedReceiver<BlockImportNotification<Block>>;
+
+/// A stream of block finality notifications.
+pub type FinalityNotifications<Block> = mpsc::UnboundedReceiver<FinalityNotification<Block>>;
 
 /// Substrate Client
 pub struct Client<B, E, Block> where Block: BlockT {
@@ -48,6 +51,7 @@ pub struct Client<B, E, Block> where Block: BlockT {
 	executor: E,
 	storage_notifications: Mutex<StorageNotifications<Block>>,
 	import_notification_sinks: Mutex<Vec<mpsc::UnboundedSender<BlockImportNotification<Block>>>>,
+	finality_notification_sinks: Mutex<Vec<mpsc::UnboundedSender<FinalityNotification<Block>>>>,
 	import_lock: Mutex<()>,
 	importing_block: RwLock<Option<Block::Hash>>, // holds the block hash currently being imported. TODO: replace this with block queue
 	execution_strategy: ExecutionStrategy,
@@ -55,8 +59,13 @@ pub struct Client<B, E, Block> where Block: BlockT {
 
 /// A source of blockchain evenets.
 pub trait BlockchainEvents<Block: BlockT> {
-	/// Get block import event stream.
-	fn import_notification_stream(&self) -> BlockchainEventStream<Block>;
+	/// Get block import event stream. Not guaranteed to be fired for every
+	/// imported block.
+	fn import_notification_stream(&self) -> ImportNotifications<Block>;
+
+	/// Get a stream of finality notifications. Not guaranteed to be fired for every
+	/// finalized block.
+	fn finality_notification_stream(&self) -> FinalityNotifications<Block>;
 
 	/// Get storage changes event stream.
 	///
@@ -146,6 +155,15 @@ pub struct BlockImportNotification<Block: BlockT> {
 	pub is_new_best: bool,
 }
 
+/// Summary of a finalized block.
+#[derive(Clone, Debug)]
+pub struct FinalityNotification<Block: BlockT> {
+	/// Imported block header hash.
+	pub hash: Block::Hash,
+	/// Imported block header.
+	pub header: Block::Header,
+}
+
 /// A header paired with a justification which has already been checked.
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub struct JustifiedHeader<Block: BlockT> {
@@ -208,6 +226,7 @@ impl<B, E, Block> Client<B, E, Block> where
 			executor,
 			storage_notifications: Default::default(),
 			import_notification_sinks: Default::default(),
+			finality_notification_sinks: Default::default(),
 			import_lock: Default::default(),
 			importing_block: Default::default(),
 			execution_strategy,
@@ -436,6 +455,25 @@ impl<B, E, Block> Client<B, E, Block> where
 			blockchain::BlockStatus::Unknown => {},
 		}
 
+		let (last_best, last_best_number) = {
+			let info = self.backend.blockchain().info()?;
+			(info.best_hash, info.best_number)
+		};
+
+		// this is a fairly arbitrary choice of where to draw the line on making notifications,
+		// but the general goal is to only make notifications when we are already fully synced
+		// and get a new chain head.
+		let make_notifications = match origin {
+			BlockOrigin::NetworkBroadcast | BlockOrigin::Own | BlockOrigin::ConsensusBroadcast => true,
+			BlockOrigin::Genesis | BlockOrigin::NetworkInitialSync | BlockOrigin::File => false,
+		};
+
+		// ensure parent block is finalized to maintain invariant that
+		// finality is called sequentially.
+		if finalized {
+			self.apply_finality(parent_hash, last_best, make_notifications)?;
+		}
+
 		let mut transaction = self.backend.begin_operation(BlockId::Hash(parent_hash))?;
 		let (storage_update, changes_update, storage_changes) = match transaction.state()? {
 			Some(transaction_state) => {
@@ -470,7 +508,8 @@ impl<B, E, Block> Client<B, E, Block> where
 			None => (None, None, None)
 		};
 
-		let is_new_best = header.number() == &(self.backend.blockchain().info()?.best_number + One::one());
+		// TODO: non longest-chain rule.
+		let is_new_best = finalized || header.number() > &last_best_number;
 		let leaf_state = if finalized {
 			::backend::NewBlockState::Final
 		} else if is_new_best {
@@ -498,26 +537,112 @@ impl<B, E, Block> Client<B, E, Block> where
 		}
 		self.backend.commit_operation(transaction)?;
 
-		if origin == BlockOrigin::NetworkBroadcast || origin == BlockOrigin::Own || origin == BlockOrigin::ConsensusBroadcast {
-
+		if make_notifications {
 			if let Some(storage_changes) = storage_changes {
 				// TODO [ToDr] How to handle re-orgs? Should we re-emit all storage changes?
 				self.storage_notifications.lock()
 					.trigger(&hash, storage_changes);
 			}
 
+			if finalized {
+				let notification = FinalityNotification::<Block> {
+					hash,
+					header: header.clone(),
+				};
+
+				self.finality_notification_sinks.lock()
+					.retain(|sink| sink.unbounded_send(notification.clone()).is_ok());
+			}
+
 			let notification = BlockImportNotification::<Block> {
-				hash: hash,
-				origin: origin,
-				header: header,
-				is_new_best: is_new_best,
+				hash,
+				origin,
+				header,
+				is_new_best,
 			};
+
 			self.import_notification_sinks.lock()
 				.retain(|sink| sink.unbounded_send(notification.clone()).is_ok());
 		}
+
 		Ok(ImportResult::Queued)
 	}
 
+	/// Finalizes all blocks up to given.
+	fn apply_finality(&self, block: Block::Hash, best_block: Block::Hash, notify: bool) -> error::Result<()> {
+		// find tree route from last finalized to given block.
+		let last_finalized = self.backend.blockchain().last_finalized()?;
+
+		if block == last_finalized { return Ok(()) }
+		let route_from_finalized = ::blockchain::tree_route(
+			self.backend.blockchain(),
+			BlockId::Hash(last_finalized),
+			BlockId::Hash(block),
+		)?;
+
+		if let Some(retracted) = route_from_finalized.retracted().get(0) {
+			warn!("Safety violation: attempted to revert finalized block {:?} which is not in the \
+				same chain as last finalized {:?}", retracted, last_finalized);
+
+			bail!(error::ErrorKind::NotInFinalizedChain);
+		}
+
+		let route_from_best = ::blockchain::tree_route(
+			self.backend.blockchain(),
+			BlockId::Hash(best_block),
+			BlockId::Hash(block),
+		)?;
+
+		// if the block is not a direct ancestor of the current best chain,
+		// then some other block is the common ancestor.
+		if route_from_best.common_block().hash != block {
+			// TODO: reorganize best block to be the best chain containing
+			// `block`.
+		}
+
+		for finalize_new in route_from_finalized.enacted() {
+			self.backend.finalize_block(BlockId::Hash(finalize_new.hash))?;
+		}
+
+		if notify {
+			// sometimes when syncing, tons of blocks can be finalized at once.
+			// we'll send notifications spuriously in that case.
+			const MAX_TO_NOTIFY: usize = 256;
+			let enacted = route_from_finalized.enacted();
+			let start = enacted.len() - ::std::cmp::min(enacted.len(), MAX_TO_NOTIFY);
+			let mut sinks = self.finality_notification_sinks.lock();
+			for finalized in &enacted[start..] {
+				let header = self.header(&BlockId::Hash(finalized.hash))?
+					.expect("header already known to exist in DB because it is indicated in the tree route; qed");
+				let notification = FinalityNotification {
+					header,
+					hash: finalized.hash,
+				};
+
+				sinks.retain(|sink| sink.unbounded_send(notification.clone()).is_ok());
+			}
+		}
+
+		Ok(())
+	}
+
+	/// Finalize a block. This will implicitly finalize all blocks up to it and
+	/// fire finality notifications.
+	///
+	/// Pass a flag to indicate whether finality notifications should be propagated.
+	/// This is usually tied to some synchronization state, where we don't send notifications
+	/// while performing major synchronization work.
+	pub fn finalize_block(&self, id: BlockId<Block>, notify: bool) -> error::Result<()> {
+		let last_best = self.backend.blockchain().info()?.best_hash;
+		let to_finalize_hash = match id {
+			BlockId::Hash(h) => h,
+			BlockId::Number(n) => self.backend.blockchain().hash(n)?
+				.ok_or_else(|| error::ErrorKind::UnknownBlock(format!("No block with number {:?}", n)))?,
+		};
+
+		self.apply_finality(to_finalize_hash, last_best, notify)
+	}
+
 	/// Attempts to revert the chain by `n` blocks. Returns the number of blocks that were
 	/// successfully reverted.
 	pub fn revert(&self, n: NumberFor<Block>) -> error::Result<NumberFor<Block>> {
@@ -681,12 +806,18 @@ where
 	Block: BlockT,
 {
 	/// Get block import event stream.
-	fn import_notification_stream(&self) -> BlockchainEventStream<Block> {
+	fn import_notification_stream(&self) -> ImportNotifications<Block> {
 		let (sink, stream) = mpsc::unbounded();
 		self.import_notification_sinks.lock().push(sink);
 		stream
 	}
 
+	fn finality_notification_stream(&self) -> FinalityNotifications<Block> {
+		let (sink, stream) = mpsc::unbounded();
+		self.finality_notification_sinks.lock().push(sink);
+		stream
+	}
+
 	/// Get storage changes event stream.
 	fn storage_changes_notification_stream(&self, filter_keys: Option<&[StorageKey]>) -> error::Result<StorageEventStream<Block::Hash>> {
 		Ok(self.storage_notifications.lock().listen(filter_keys))
diff --git a/substrate/core/client/src/error.rs b/substrate/core/client/src/error.rs
index 05429b9566..3dd79d79e6 100644
--- a/substrate/core/client/src/error.rs
+++ b/substrate/core/client/src/error.rs
@@ -99,6 +99,18 @@ error_chain! {
 			description("Error decoding call result")
 			display("Error decoding call result of {}", method)
 		}
+
+		/// Last finalized block not parent of current.
+		NonSequentialFinalization(s: String) {
+			description("Did not finalize blocks in sequential order."),
+			display("Did not finalize blocks in sequential order."),
+		}
+
+		/// Safety violation: new best block not descendent of last finalized.
+		NotInFinalizedChain {
+			description("Potential long-range attack: block not in finalized chain."),
+			display("Potential long-range attack: block not in finalized chain."),
+		}
 	}
 }
 
diff --git a/substrate/core/client/src/in_mem.rs b/substrate/core/client/src/in_mem.rs
index ef1ea954df..4c321db09c 100644
--- a/substrate/core/client/src/in_mem.rs
+++ b/substrate/core/client/src/in_mem.rs
@@ -221,6 +221,7 @@ impl<Block: BlockT> HeaderBackend<Block> for Blockchain<Block> {
 			best_hash: storage.best_hash,
 			best_number: storage.best_number,
 			genesis_hash: storage.genesis_hash,
+			finalized_hash: storage.finalized_hash,
 		})
 	}
 
diff --git a/substrate/core/client/src/lib.rs b/substrate/core/client/src/lib.rs
index 37f1f2ba6b..c2467bc492 100644
--- a/substrate/core/client/src/lib.rs
+++ b/substrate/core/client/src/lib.rs
@@ -61,7 +61,7 @@ pub use blockchain::Info as ChainInfo;
 pub use call_executor::{CallResult, CallExecutor, LocalCallExecutor};
 pub use client::{
 	new_in_mem,
-	BlockBody, BlockStatus, BlockOrigin, BlockchainEventStream, BlockchainEvents,
+	BlockBody, BlockStatus, BlockOrigin, ImportNotifications, FinalityNotifications, BlockchainEvents,
 	Client, ClientInfo, ChainHead,
 	ImportResult, JustifiedHeader,
 };
diff --git a/substrate/core/network/src/chain.rs b/substrate/core/network/src/chain.rs
index 5d162cef78..be8e4ba18d 100644
--- a/substrate/core/network/src/chain.rs
+++ b/substrate/core/network/src/chain.rs
@@ -26,7 +26,14 @@ use primitives::{Blake2Hasher, RlpCodec};
 /// Local client abstraction for the network.
 pub trait Client<Block: BlockT>: Send + Sync {
 	/// Import a new block. Parent is supposed to be existing in the blockchain.
-	fn import(&self, origin: BlockOrigin, header: Block::Header, justification: Justification<Block::Hash>, body: Option<Vec<Block::Extrinsic>>) -> Result<ImportResult, Error>;
+	fn import(
+		&self,
+		origin: BlockOrigin,
+		header: Block::Header,
+		justification: Justification<Block::Hash>,
+		body: Option<Vec<Block::Extrinsic>>,
+		finalized: bool,
+	) -> Result<ImportResult, Error>;
 
 	/// Get blockchain info.
 	fn info(&self) -> Result<ClientInfo<Block>, Error>;
@@ -61,10 +68,17 @@ impl<B, E, Block> Client<Block> for SubstrateClient<B, E, Block> where
 	E: CallExecutor<Block, Blake2Hasher, RlpCodec> + Send + Sync + 'static,
 	Block: BlockT,
 {
-	fn import(&self, origin: BlockOrigin, header: Block::Header, justification: Justification<Block::Hash>, body: Option<Vec<Block::Extrinsic>>) -> Result<ImportResult, Error> {
-		// TODO: defer justification check and non-instant finality.
+	fn import(
+		&self,
+		origin: BlockOrigin,
+		header: Block::Header,
+		justification: Justification<Block::Hash>,
+		body: Option<Vec<Block::Extrinsic>>,
+		finalized: bool,
+	) -> Result<ImportResult, Error> {
+		// TODO: defer justification check and add finality.
 		let justified_header = self.check_justification(header, justification.into())?;
-		(self as &SubstrateClient<B, E, Block>).import_block(origin, justified_header, body, true)
+		(self as &SubstrateClient<B, E, Block>).import_block(origin, justified_header, body, finalized)
 	}
 
 	fn info(&self) -> Result<ClientInfo<Block>, Error> {
diff --git a/substrate/core/network/src/import_queue.rs b/substrate/core/network/src/import_queue.rs
index 647070383b..1e1149dc6f 100644
--- a/substrate/core/network/src/import_queue.rs
+++ b/substrate/core/network/src/import_queue.rs
@@ -59,6 +59,7 @@ pub struct ImportQueueStatus<B: BlockT> {
 pub struct AsyncImportQueue<B: BlockT> {
 	handle: Mutex<Option<::std::thread::JoinHandle<()>>>,
 	data: Arc<AsyncImportQueueData<B>>,
+	instant_finality: bool,
 }
 
 /// Locks order: queue, queue_blocks, best_importing_number
@@ -71,10 +72,11 @@ struct AsyncImportQueueData<B: BlockT> {
 }
 
 impl<B: BlockT> AsyncImportQueue<B> {
-	pub fn new() -> Self {
+	pub fn new(instant_finality: bool) -> Self {
 		Self {
 			handle: Mutex::new(None),
 			data: Arc::new(AsyncImportQueueData::new()),
+			instant_finality,
 		}
 	}
 
@@ -82,8 +84,9 @@ impl<B: BlockT> AsyncImportQueue<B> {
 		debug_assert!(self.handle.lock().is_none());
 
 		let qdata = self.data.clone();
+		let instant_finality = self.instant_finality;
 		*self.handle.lock() = Some(::std::thread::Builder::new().name("ImportQueue".into()).spawn(move || {
-			import_thread(sync, service, chain, qdata)
+			import_thread(sync, service, chain, qdata, instant_finality)
 		}).map_err(|err| Error::from(ErrorKind::Io(err)))?);
 		Ok(())
 	}
@@ -159,7 +162,13 @@ impl<B: BlockT> Drop for AsyncImportQueue<B> {
 }
 
 /// Blocks import thread.
-fn import_thread<B: BlockT, E: ExecuteInContext<B>>(sync: Weak<RwLock<ChainSync<B>>>, service: Weak<E>, chain: Weak<Client<B>>, qdata: Arc<AsyncImportQueueData<B>>) {
+fn import_thread<B: BlockT, E: ExecuteInContext<B>>(
+	sync: Weak<RwLock<ChainSync<B>>>,
+	service: Weak<E>,
+	chain: Weak<Client<B>>,
+	qdata: Arc<AsyncImportQueueData<B>>,
+	instant_finality: bool,
+) {
 	trace!(target: "sync", "Starting import thread");
 	loop {
 		if qdata.is_stopping.load(Ordering::SeqCst) {
@@ -181,7 +190,12 @@ fn import_thread<B: BlockT, E: ExecuteInContext<B>>(sync: Weak<RwLock<ChainSync<
 		match (sync.upgrade(), service.upgrade(), chain.upgrade()) {
 			(Some(sync), Some(service), Some(chain)) => {
 				let blocks_hashes: Vec<B::Hash> = new_blocks.1.iter().map(|b| b.block.hash.clone()).collect();
-				if !import_many_blocks(&mut SyncLink::Indirect(&sync, &*chain, &*service), Some(&*qdata), new_blocks) {
+				if !import_many_blocks(
+					&mut SyncLink::Indirect(&sync, &*chain, &*service),
+					Some(&*qdata),
+					new_blocks,
+					instant_finality,
+				) {
 					break;
 				}
 
@@ -246,7 +260,8 @@ enum BlockImportError {
 fn import_many_blocks<'a, B: BlockT>(
 	link: &mut SyncLinkApi<B>,
 	qdata: Option<&AsyncImportQueueData<B>>,
-	blocks: (BlockOrigin, Vec<BlockData<B>>)
+	blocks: (BlockOrigin, Vec<BlockData<B>>),
+	instant_finality: bool,
 ) -> bool
 {
 	let (blocks_origin, blocks) = blocks;
@@ -265,7 +280,12 @@ fn import_many_blocks<'a, B: BlockT>(
 
 	// Blocks in the response/drain should be in ascending order.
 	for block in blocks {
-		let import_result = import_single_block(link.chain(), blocks_origin.clone(), block);
+		let import_result = import_single_block(
+			link.chain(),
+			blocks_origin.clone(),
+			block,
+			instant_finality,
+		);
 		let is_import_failed = import_result.is_err();
 		imported += process_import_result(link, import_result);
 		if is_import_failed {
@@ -287,7 +307,8 @@ fn import_many_blocks<'a, B: BlockT>(
 fn import_single_block<B: BlockT>(
 	chain: &Client<B>,
 	block_origin: BlockOrigin,
-	block: BlockData<B>
+	block: BlockData<B>,
+	instant_finality: bool,
 ) -> Result<BlockImportResult<B::Hash, <<B as BlockT>::Header as HeaderT>::Number>, BlockImportError>
 {
 	let origin = block.origin;
@@ -303,6 +324,7 @@ fn import_single_block<B: BlockT>(
 				header,
 				justification,
 				block.body,
+				instant_finality,
 			);
 			match result {
 				Ok(ImportResult::AlreadyInChain) => {
@@ -436,7 +458,8 @@ pub mod tests {
 	use super::*;
 
 	/// Blocks import queue that is importing blocks in the same thread.
-	pub struct SyncImportQueue;
+	/// The boolean value indicates whether blocks should be imported without instant finality.
+	pub struct SyncImportQueue(pub bool);
 	struct DummyExecuteInContext;
 
 	impl<B: 'static + BlockT> ExecuteInContext<B> for DummyExecuteInContext {
@@ -460,7 +483,7 @@ pub mod tests {
 		}
 
 		fn import_blocks(&self, sync: &mut ChainSync<B>, protocol: &mut Context<B>, blocks: (BlockOrigin, Vec<BlockData<B>>)) {
-			import_many_blocks(&mut SyncLink::Direct::<_, DummyExecuteInContext>(sync, protocol), None, blocks);
+			import_many_blocks(&mut SyncLink::Direct::<_, DummyExecuteInContext>(sync, protocol), None, blocks, self.0);
 		}
 	}
 
@@ -518,27 +541,39 @@ pub mod tests {
 	#[test]
 	fn import_single_good_block_works() {
 		let (_, hash, number, block) = prepare_good_block();
-		assert_eq!(import_single_block(&test_client::new(), BlockOrigin::File, block), Ok(BlockImportResult::ImportedUnknown(hash, number)));
+		assert_eq!(
+			import_single_block(&test_client::new(), BlockOrigin::File, block, true),
+			Ok(BlockImportResult::ImportedUnknown(hash, number))
+		);
 	}
 
 	#[test]
 	fn import_single_good_known_block_is_ignored() {
 		let (client, hash, number, block) = prepare_good_block();
-		assert_eq!(import_single_block(&client, BlockOrigin::File, block), Ok(BlockImportResult::ImportedKnown(hash, number)));
+		assert_eq!(
+			import_single_block(&client, BlockOrigin::File, block, true),
+			Ok(BlockImportResult::ImportedKnown(hash, number))
+		);
 	}
 
 	#[test]
 	fn import_single_good_block_without_header_fails() {
 		let (_, _, _, mut block) = prepare_good_block();
 		block.block.header = None;
-		assert_eq!(import_single_block(&test_client::new(), BlockOrigin::File, block), Err(BlockImportError::Disconnect(0)));
+		assert_eq!(
+			import_single_block(&test_client::new(), BlockOrigin::File, block, true),
+			Err(BlockImportError::Disconnect(0))
+		);
 	}
 
 	#[test]
 	fn import_single_good_block_without_justification_fails() {
 		let (_, _, _, mut block) = prepare_good_block();
 		block.block.justification = None;
-		assert_eq!(import_single_block(&test_client::new(), BlockOrigin::File, block), Err(BlockImportError::Disconnect(0)));
+		assert_eq!(
+			import_single_block(&test_client::new(), BlockOrigin::File, block, true),
+			Err(BlockImportError::Disconnect(0))
+		);
 	}
 
 	#[test]
@@ -579,12 +614,17 @@ pub mod tests {
 		let (_, _, _, block) = prepare_good_block();
 		let qdata = AsyncImportQueueData::new();
 		qdata.is_stopping.store(true, Ordering::SeqCst);
-		assert!(!import_many_blocks(&mut TestLink::new(), Some(&qdata), (BlockOrigin::File, vec![block.clone(), block])));
+		assert!(!import_many_blocks(
+			&mut TestLink::new(),
+			Some(&qdata),
+			(BlockOrigin::File, vec![block.clone(), block]),
+			true
+		));
 	}
 
 	#[test]
 	fn async_import_queue_drops() {
-		let queue = AsyncImportQueue::new();
+		let queue = AsyncImportQueue::new(true);
 		let service = Arc::new(DummyExecutor);
 		let chain = Arc::new(test_client::new());
 		queue.start(Weak::new(), Arc::downgrade(&service), Arc::downgrade(&chain) as Weak<Client<Block>>).unwrap();
diff --git a/substrate/core/network/src/service.rs b/substrate/core/network/src/service.rs
index 4b27c843d9..c900cae928 100644
--- a/substrate/core/network/src/service.rs
+++ b/substrate/core/network/src/service.rs
@@ -165,7 +165,8 @@ impl<B: BlockT + 'static, S: Specialization<B>, H: ExHashT> Service<B, S, H> {
 	/// Creates and register protocol with the network service
 	pub fn new(params: Params<B, S, H>, protocol_id: ProtocolId) -> Result<Arc<Service<B, S, H>>, Error> {
 		let chain = params.chain.clone();
-		let import_queue = Arc::new(AsyncImportQueue::new());
+		// TODO: non-insatnt finality.
+		let import_queue = Arc::new(AsyncImportQueue::new(true));
 		let handler = Arc::new(ProtocolHandler {
 			protocol: Protocol::new(
 				params.config,
diff --git a/substrate/core/network/src/specialization.rs b/substrate/core/network/src/specialization.rs
index 3c04a36705..1c2d2d7274 100644
--- a/substrate/core/network/src/specialization.rs
+++ b/substrate/core/network/src/specialization.rs
@@ -44,5 +44,6 @@ pub trait Specialization<B: BlockT>: Send + Sync + 'static {
 	fn maintain_peers(&mut self, _ctx: &mut Context<B>) { }
 
 	/// Called when a block is _imported_ at the head of the chain (not during major sync).
+	/// Not guaranteed to be called for every block, but will be most of the after major sync.
 	fn on_block_imported(&mut self, _ctx: &mut Context<B>, _hash: B::Hash, _header: &B::Header) { }
 }
diff --git a/substrate/core/network/src/test/mod.rs b/substrate/core/network/src/test/mod.rs
index 7a78dd28fc..0fa5196a03 100644
--- a/substrate/core/network/src/test/mod.rs
+++ b/substrate/core/network/src/test/mod.rs
@@ -173,8 +173,8 @@ impl Peer {
 	fn flush(&self) {
 	}
 
-	fn generate_blocks<F>(&self, count: usize, mut edit_block: F) 
-	where F: FnMut(&mut BlockBuilder<test_client::Backend, test_client::Executor, Block, Blake2Hasher, RlpCodec>) 
+	fn generate_blocks<F>(&self, count: usize, mut edit_block: F)
+	where F: FnMut(&mut BlockBuilder<test_client::Backend, test_client::Executor, Block, Blake2Hasher, RlpCodec>)
 	{
 		for _ in 0 .. count {
 			let mut builder = self.client.new_block().unwrap();
@@ -246,7 +246,7 @@ impl TestNet {
 	pub fn add_peer(&mut self, config: &ProtocolConfig) {
 		let client = Arc::new(test_client::new());
 		let tx_pool = Arc::new(EmptyTransactionPool);
-		let import_queue = Arc::new(SyncImportQueue);
+		let import_queue = Arc::new(SyncImportQueue(false));
 		let sync = Protocol::new(config.clone(), client.clone(), import_queue, None, tx_pool, DummySpecialization).unwrap();
 		self.peers.push(Arc::new(Peer {
 			sync: sync,
diff --git a/substrate/core/test-client/src/client_ext.rs b/substrate/core/test-client/src/client_ext.rs
index 624b360f0b..7d1a9b433f 100644
--- a/substrate/core/test-client/src/client_ext.rs
+++ b/substrate/core/test-client/src/client_ext.rs
@@ -18,7 +18,7 @@
 
 use client::{self, Client};
 use keyring::Keyring;
-use runtime_primitives::StorageMap;
+use runtime_primitives::{generic::BlockId, StorageMap};
 use runtime::genesismap::{GenesisConfig, additional_storage_with_genesis};
 use executor::NativeExecutor;
 use runtime;
@@ -30,9 +30,12 @@ pub trait TestClient {
 	/// Crates new client instance for tests.
 	fn new_for_tests() -> Self;
 
-	/// Justify and import block to the chain. Instant finality.
+	/// Justify and import block to the chain. No finality.
 	fn justify_and_import(&self, origin: client::BlockOrigin, block: runtime::Block) -> client::error::Result<()>;
 
+	/// Finalize a block.
+	fn finalize_block(&self, id: BlockId<runtime::Block>) -> client::error::Result<()>;
+
 	/// Returns hash of the genesis block.
 	fn genesis_hash(&self) -> runtime::Hash;
 }
@@ -45,11 +48,15 @@ impl TestClient for Client<Backend, Executor, runtime::Block> {
 	fn justify_and_import(&self, origin: client::BlockOrigin, block: runtime::Block) -> client::error::Result<()> {
 		let justification = fake_justify(&block.header);
 		let justified = self.check_justification(block.header, justification)?;
-		self.import_block(origin, justified, Some(block.extrinsics), true)?;
+		self.import_block(origin, justified, Some(block.extrinsics), false)?;
 
 		Ok(())
 	}
 
+	fn finalize_block(&self, id: BlockId<runtime::Block>) -> client::error::Result<()> {
+		self.finalize_block(id, true)
+	}
+
 	fn genesis_hash(&self) -> runtime::Hash {
 		self.block_hash(0).unwrap().unwrap()
 	}