diff --git a/substrate/wasm-runtime/polkadot/src/lib.rs b/substrate/wasm-runtime/polkadot/src/lib.rs
index 6a6201945d..500594e31a 100644
--- a/substrate/wasm-runtime/polkadot/src/lib.rs
+++ b/substrate/wasm-runtime/polkadot/src/lib.rs
@@ -15,15 +15,10 @@ pub use support::{primitives, function, environment, storage};
 #[cfg(test)]
 pub use support::{testing, statichex};
 
-
-#[allow(unused_imports)]		// TODO: remove in due course
 use runtime_support::Vec;
 use slicable::Slicable;
 use primitives::{Block, UncheckedTransaction};
 
-// TODO: add externals for:
-// - trie rooting
-
 pub fn execute_block(input: Vec<u8>) -> Vec<u8> {
 	runtime::system::execute_block(Block::from_slice(&input).unwrap());
 	Vec::new()
diff --git a/substrate/wasm-runtime/polkadot/src/runtime/consensus.rs b/substrate/wasm-runtime/polkadot/src/runtime/consensus.rs
index 0d2531cea4..f8991e8871 100644
--- a/substrate/wasm-runtime/polkadot/src/runtime/consensus.rs
+++ b/substrate/wasm-runtime/polkadot/src/runtime/consensus.rs
@@ -2,6 +2,7 @@ use runtime_support::Vec;
 use keyedvec::KeyedVec;
 use storage::Storage;
 use primitives::{AccountID, SessionKey, BlockNumber};
+use runtime::{system, staking};
 
 pub fn set_authority(index: u32, authority: AccountID) {
 	authority.store(&index.to_keyed_vec(b"con\0aut\0"));
@@ -32,41 +33,3 @@ pub fn set_authorities(authorities: &[AccountID]) {
 	set_authority_count(authorities.len() as u32);
 	authorities.iter().enumerate().for_each(|(v, &i)| set_authority(v as u32, i));
 }
-
-/// Get the current set of validators. These are the long-term identifiers for the validators
-/// and will be mapped to a session key with the most recent `set_next_session_key`.
-pub fn validators() -> Vec<AccountID> {
-	unimplemented!()
-}
-
-/// Set the current set of validators.
-///
-/// Called by staking::next_era() only.
-pub fn set_validators(_new: &[AccountID]) {
-	unimplemented!()
-}
-
-/// The number of blocks in each session.
-pub fn session_length() -> BlockNumber {
-	Storage::into(b"con\0bps")
-}
-
-/// Sets the session key of `_transactor` to `_session`. This doesn't take effect until the next
-/// session.
-pub fn set_session_key(_transactor: &AccountID, _session: &AccountID) {
-	unimplemented!()
-}
-
-/// Move onto next session: register the new authority set.
-pub fn next_session() {
-	// TODO: Call set_authorities().
-	unimplemented!()
-}
-
-/// Hook to be called prior to transaction processing.
-pub fn pre_transactions() {}
-
-/// Hook to be called after to transaction processing.
-pub fn post_transactions() {
-	// TODO: check block number and call next_session if necessary.
-}
diff --git a/substrate/wasm-runtime/polkadot/src/runtime/mod.rs b/substrate/wasm-runtime/polkadot/src/runtime/mod.rs
index c73e338a91..8e60ec431c 100644
--- a/substrate/wasm-runtime/polkadot/src/runtime/mod.rs
+++ b/substrate/wasm-runtime/polkadot/src/runtime/mod.rs
@@ -6,3 +6,5 @@ pub mod consensus;
 pub mod staking;
 #[allow(unused)]
 pub mod timestamp;
+#[allow(unused)]
+pub mod session;
diff --git a/substrate/wasm-runtime/polkadot/src/runtime/session.rs b/substrate/wasm-runtime/polkadot/src/runtime/session.rs
new file mode 100644
index 0000000000..f72ef0b5c6
--- /dev/null
+++ b/substrate/wasm-runtime/polkadot/src/runtime/session.rs
@@ -0,0 +1,51 @@
+use runtime_support::Vec;
+use keyedvec::KeyedVec;
+use storage::Storage;
+use primitives::{AccountID, SessionKey, BlockNumber};
+use runtime::{system, staking, consensus};
+
+/// Get the current set of validators. These are the long-term identifiers for the validators
+/// and will be mapped to a session key with the most recent `set_next_session_key`.
+pub fn validators() -> Vec<AccountID> {
+	consensus::authorities()
+}
+
+/// Set the current set of validators.
+///
+/// Called by staking::next_era() only.
+pub fn set_validators(new: &[AccountID]) {
+	consensus::set_authorities(new);
+}
+
+/// The number of blocks in each session.
+pub fn length() -> BlockNumber {
+	Storage::into(b"con\0bps")
+}
+
+/// Sets the session key of `_transactor` to `_session`. This doesn't take effect until the next
+/// session.
+pub fn set_key(_transactor: &AccountID, _session: &AccountID) {
+	// TODO: record the new session key for `_transactor`, ready for the next session.
+}
+
+/// Move onto next session: register the new authority set.
+pub fn next_session() {
+	// TODO: Call set_authorities() with any new authorities.
+}
+
+/// Hook to be called prior to transaction processing.
+pub fn pre_transactions() {
+	staking::pre_transactions();
+}
+
+/// Hook to be called after to transaction processing.
+pub fn post_transactions() {
+	staking::post_transactions();
+
+	// do this last, after the staking system has had chance to switch out the authorities for the
+	// new set.
+	// check block number and call next_session if necessary.
+	if system::block_number() % length() == 0 {
+		next_session();
+	}
+}
diff --git a/substrate/wasm-runtime/polkadot/src/runtime/staking.rs b/substrate/wasm-runtime/polkadot/src/runtime/staking.rs
index 6e22cfcd5f..366ad0668b 100644
--- a/substrate/wasm-runtime/polkadot/src/runtime/staking.rs
+++ b/substrate/wasm-runtime/polkadot/src/runtime/staking.rs
@@ -1,11 +1,11 @@
 use keyedvec::KeyedVec;
 use storage::Storage;
 use primitives::{BlockNumber, Balance, AccountID};
-use runtime::consensus;
+use runtime::{system, session};
 
 /// The length of a staking era in blocks.
 pub fn era_length() -> BlockNumber {
-	sessions_per_era() * consensus::session_length()
+	sessions_per_era() * session::length()
 }
 
 /// The length of a staking era in sessions.
@@ -17,7 +17,8 @@ pub fn sessions_per_era() -> BlockNumber {
 ///
 /// NOTE: This always happens on a session change.
 pub fn next_era() {
-	unimplemented!()
+	// TODO: evaluate desired staking amounts and nominations and optimise to find the best
+	// combination of validators, then use session::set_validators().
 }
 
 /// The balance of a given account.
@@ -53,13 +54,14 @@ pub fn unstake(_transactor: &AccountID) {
 
 /// Hook to be called prior to transaction processing.
 pub fn pre_transactions() {
-	consensus::pre_transactions();
 }
 
 /// Hook to be called after to transaction processing.
 pub fn post_transactions() {
-	// TODO: check block number and call next_era if necessary.
-	consensus::post_transactions();
+	// check block number and call next_era if necessary.
+	if system::block_number() % era_length() == 0 {
+		next_era();
+	}
 }
 
 
diff --git a/substrate/wasm-runtime/polkadot/src/runtime/system.rs b/substrate/wasm-runtime/polkadot/src/runtime/system.rs
index fe0e835608..690e1db527 100644
--- a/substrate/wasm-runtime/polkadot/src/runtime/system.rs
+++ b/substrate/wasm-runtime/polkadot/src/runtime/system.rs
@@ -3,7 +3,7 @@ use runtime_support::{Vec, swap};
 use storage::Storage;
 use keyedvec::KeyedVec;
 use environment::with_env;
-use runtime::staking;
+use runtime::session;
 
 /// The current block number being processed. Set by `execute_block`.
 pub fn block_number() -> BlockNumber {
@@ -40,20 +40,24 @@ pub fn execute_block(mut block: Block) {
 	);
 
 	// TODO: check transaction trie root represents the transactions.
+	// this requires non-trivial changes to the externals API or compiling trie rooting into wasm
+	// so will wait until a little later.
 
 	// store the header hash in storage.
 	let header_hash_key = header.number.to_keyed_vec(b"sys\0old\0");
 	header.keccak256().store(&header_hash_key);
 
 	// execute transactions
-	staking::pre_transactions();
+	session::pre_transactions();
 	block.transactions.iter().for_each(execute_transaction);
-	staking::post_transactions();
+	session::post_transactions();
 
 	// any final checks
 	final_checks(&block);
 
-	// TODO: check storage root somehow
+	// TODO: check storage root.
+	// this requires non-trivial changes to the externals API or compiling trie rooting into wasm
+	// so will wait until a little later.
 }
 
 fn final_checks(_block: &Block) {
diff --git a/substrate/wasm-runtime/polkadot/src/support/function.rs b/substrate/wasm-runtime/polkadot/src/support/function.rs
index ff54938d47..995ec3c459 100644
--- a/substrate/wasm-runtime/polkadot/src/support/function.rs
+++ b/substrate/wasm-runtime/polkadot/src/support/function.rs
@@ -1,4 +1,4 @@
-use runtime::{staking, consensus};
+use runtime::{staking, session};
 use primitives::AccountID;
 use streamreader::StreamReader;
 
@@ -9,7 +9,7 @@ pub enum Function {
 	StakingStake,
 	StakingUnstake,
 	StakingTransferStake,
-	ConsensusSetSessionKey,
+	SessionSetKey,
 }
 
 impl Function {
@@ -18,7 +18,7 @@ impl Function {
 			x if x == Function::StakingStake as u8 => Some(Function::StakingStake),
 			x if x == Function::StakingUnstake as u8 => Some(Function::StakingUnstake),
 			x if x == Function::StakingTransferStake as u8 => Some(Function::StakingTransferStake),
-			x if x == Function::ConsensusSetSessionKey as u8 => Some(Function::ConsensusSetSessionKey),
+			x if x == Function::SessionSetKey as u8 => Some(Function::SessionSetKey),
 			_ => None,
 		}
 	}
@@ -40,9 +40,9 @@ impl Function {
 				let value = params.read().unwrap();
 				staking::transfer_stake(transactor, &dest, value);
 			}
-			Function::ConsensusSetSessionKey => {
+			Function::SessionSetKey => {
 				let session = params.read().unwrap();
-				consensus::set_session_key(transactor, &session);
+				session::set_key(transactor, &session);
 			}
 		}
 	}