diff --git a/substrate/substrate/client/src/block_builder.rs b/substrate/substrate/client/src/block_builder.rs
new file mode 100644
index 0000000000..ebd8ec6c47
--- /dev/null
+++ b/substrate/substrate/client/src/block_builder.rs
@@ -0,0 +1,89 @@
+// Copyright 2017 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Utility struct to build a block.
+
+use std::vec::Vec;
+use codec::{Joiner, Slicable};
+use state_machine::{self, CodeExecutor};
+use primitives::{Header, Block};
+use primitives::block::Transaction;
+use {backend, error, BlockId, BlockStatus, Client};
+use triehash::ordered_trie_root;
+
+/// Utility for building new (valid) blocks from a stream of transactions.
+pub struct BlockBuilder<B, E> where
+	B: backend::Backend,
+	E: CodeExecutor + Clone,
+	error::Error: From<<<B as backend::Backend>::State as state_machine::backend::Backend>::Error>,
+{
+	header: Header,
+	transactions: Vec<Transaction>,
+	executor: E,
+	state: B::State,
+	changes: state_machine::OverlayedChanges,
+}
+
+impl<B, E> BlockBuilder<B, E> where
+	B: backend::Backend,
+	E: CodeExecutor + Clone,
+	error::Error: From<<<B as backend::Backend>::State as state_machine::backend::Backend>::Error>,
+{
+	/// Create a new instance of builder from the given client.
+	pub fn new(client: &Client<B, E>) -> error::Result<Self> {
+		let best = (client.info().map(|i| i.chain.best_number)?..1)
+			.find(|&n| if let Ok(BlockStatus::InChain) = client.block_status(&BlockId::Number(n))
+				{ true } else { false })
+			.unwrap_or(0);
+
+		Ok(BlockBuilder {
+			header: Header {
+				number: best + 1,
+				parent_hash: client.block_hash(best)?.expect("We already ascertained this is InChain before; qed"),
+				state_root: Default::default(),
+				transaction_root: Default::default(),
+				digest: Default::default(),
+			},
+			transactions: Default::default(),
+			executor: client.clone_executor(),
+			state: client.state_at(&BlockId::Number(best))?,
+			changes: Default::default(),
+		})
+	}
+
+	/// Push a transaction onto the block's list of transactions. This will ensure the transaction
+	/// can be validly executed (by executing it); if it is invalid, it'll be returned along with
+	/// the error. Otherwise, it will return a mutable reference to self (in order to chain).
+	pub fn push(&mut self, tx: Transaction) -> error::Result<()> {
+		let output = state_machine::execute(&self.state, &mut self.changes, &self.executor, "execute_transaction",
+			&vec![].and(&self.header).and(&tx))?;
+		self.header = Header::decode(&mut &output[..]).expect("Header came straight out of runtime do must be valid");
+		self.transactions.push(tx);
+		Ok(())
+	}
+
+	/// Consume the builder to return a valid `Block` containing all pushed transactions.
+	pub fn bake(mut self) -> error::Result<Block> {
+		self.header.transaction_root = ordered_trie_root(self.transactions.iter().map(Slicable::encode)).0.into();
+		let output = state_machine::execute(&self.state, &mut self.changes, &self.executor, "finalise_block",
+			&self.header.encode())?;
+		self.header = Header::decode(&mut &output[..]).expect("Header came straight out of runtime do must be valid");
+		Ok(Block {
+			header: self.header,
+			transactions: self.transactions,
+		})
+	}
+}
diff --git a/substrate/substrate/client/src/lib.rs b/substrate/substrate/client/src/lib.rs
index 530288ca29..1a7d4ceb62 100644
--- a/substrate/substrate/client/src/lib.rs
+++ b/substrate/substrate/client/src/lib.rs
@@ -39,9 +39,11 @@ pub mod blockchain;
 pub mod backend;
 pub mod in_mem;
 pub mod genesis;
+pub mod block_builder;
 
 pub use blockchain::Info as ChainInfo;
 pub use blockchain::BlockId;
+pub use block_builder::BlockBuilder;
 
 use primitives::{block, AuthorityId};
 use primitives::storage::{StorageKey, StorageData};
@@ -59,6 +61,7 @@ pub struct Client<B, E> where B: backend::Backend {
 }
 
 /// Client info
+// TODO: split queue info from chain info and amalgamate into single struct.
 #[derive(Debug)]
 pub struct ClientInfo {
 	/// Best block hash.
@@ -167,6 +170,11 @@ impl<B, E> Client<B, E> where
 		self.storage(id, &StorageKey(b":code".to_vec())).map(|data| data.0)
 	}
 
+	/// Clone a new instance of Executor.
+	pub fn clone_executor(&self) -> E where E: Clone {
+		self.executor.clone()
+	}
+
 	/// Get the current set of authorities from storage.
 	pub fn authorities_at(&self, id: &BlockId) -> error::Result<Vec<AuthorityId>> {
 		let state = self.state_at(id)?;
@@ -183,10 +191,8 @@ impl<B, E> Client<B, E> where
 	/// No changes are made.
 	pub fn call(&self, id: &BlockId, method: &str, call_data: &[u8]) -> error::Result<CallResult> {
 		let mut changes = state_machine::OverlayedChanges::default();
-		let state = self.state_at(id)?;
-
 		let return_data = state_machine::execute(
-			&state,
+			&self.state_at(id)?,
 			&mut changes,
 			&self.executor,
 			method,
@@ -195,6 +201,11 @@ impl<B, E> Client<B, E> where
 		Ok(CallResult { return_data, changes })
 	}
 
+	/// Create a new block, built on the head of the chain.
+	pub fn new_block(&self) -> error::Result<BlockBuilder<B, E>> where E: Clone {
+		BlockBuilder::new(self)
+	}
+
 	/// Queue a block for import.
 	pub fn import_block(&self, header: block::Header, body: Option<block::Body>) -> error::Result<ImportResult> {
 		// TODO: import lock
diff --git a/substrate/substrate/primitives/src/block.rs b/substrate/substrate/primitives/src/block.rs
index ba34dd085f..90882f7fc3 100644
--- a/substrate/substrate/primitives/src/block.rs
+++ b/substrate/substrate/primitives/src/block.rs
@@ -17,19 +17,18 @@
 //! Block and header type definitions.
 
 #[cfg(feature = "std")]
-use bytes;
 use rstd::vec::Vec;
+use {bytes, Hash};
 use codec::{Input, Slicable};
-use hash::H256;
 
 /// Used to refer to a block number.
 pub type Number = u64;
 
 /// Hash used to refer to a block hash.
-pub type HeaderHash = H256;
+pub type HeaderHash = Hash;
 
 /// Hash used to refer to a transaction hash.
-pub type TransactionHash = H256;
+pub type TransactionHash = Hash;
 
 /// Simple generic transaction type.
 #[derive(PartialEq, Eq, Clone)]
@@ -127,9 +126,9 @@ pub struct Header {
 	/// Block number.
 	pub number: Number,
 	/// State root after this transition.
-	pub state_root: H256,
+	pub state_root: Hash,
 	/// The root of the trie that represents this block's transactions, indexed by a 32-byte integer.
-	pub transaction_root: H256,
+	pub transaction_root: Hash,
 	/// The digest of activity on the block.
 	pub digest: Digest,
 }
diff --git a/substrate/substrate/primitives/src/lib.rs b/substrate/substrate/primitives/src/lib.rs
index 1c282fe42e..211bfce8f5 100644
--- a/substrate/substrate/primitives/src/lib.rs
+++ b/substrate/substrate/primitives/src/lib.rs
@@ -90,8 +90,10 @@ mod tests;
 
 pub use self::hash::{H160, H256};
 pub use self::uint::{U256, U512};
-
 pub use block::{Block, Header};
 
+/// General hash type.
+pub type Hash = H256;
+
 /// An identifier for an authority in the consensus algorithm. The same as ed25519::Public.
 pub type AuthorityId = [u8; 32];