diff --git a/demo/runtime/src/lib.rs b/demo/runtime/src/lib.rs
index f0c39af85d198..d78dfb56d847e 100644
--- a/demo/runtime/src/lib.rs
+++ b/demo/runtime/src/lib.rs
@@ -56,7 +56,7 @@ extern crate demo_primitives;
 use rstd::prelude::*;
 use demo_primitives::{AccountId, AccountIndex, Balance, BlockNumber, Hash, Index, SessionKey, Signature};
 use runtime_primitives::generic;
-use runtime_primitives::traits::{Convert, HasPublicAux, BlakeTwo256};
+use runtime_primitives::traits::{Convert, HasPublicAux, BlakeTwo256, Identity, DigestItem, AuthoritiesChangeDigest};
 use version::RuntimeVersion;
 
 #[cfg(any(feature = "std", test))]
@@ -94,9 +94,9 @@ impl system::Trait for Concrete {
 	type BlockNumber = BlockNumber;
 	type Hash = Hash;
 	type Hashing = BlakeTwo256;
-	type Digest = generic::Digest<Vec<u8>>;
+	type Digest = generic::Digest<ConcreteLog>;
 	type AccountId = AccountId;
-	type Header = generic::Header<BlockNumber, BlakeTwo256, Vec<u8>>;
+	type Header = generic::Header<BlockNumber, BlakeTwo256, ConcreteLog>;
 	type Event = Event;
 }
 
@@ -118,6 +118,7 @@ impl consensus::Trait for Concrete {
 	const NOTE_OFFLINE_POSITION: u32 = 1;
 	type SessionKey = SessionKey;
 	type OnOfflineValidator = Staking;
+	type ConvertSessionKeyToAuthorityId = Identity;
 }
 
 /// Consensus module for this concrete runtime.
@@ -170,6 +171,42 @@ pub type Council = council::Module<Concrete>;
 /// Council voting module for this concrete runtime.
 pub type CouncilVoting = council::voting::Module<Concrete>;
 
+/// Concrete log type.
+#[derive(Clone, PartialEq, Eq, Encode, Decode)]
+#[cfg_attr(feature = "std", derive(Debug, Serialize, Deserialize))]
+pub enum ConcreteLog {
+	/// Authority changes log.
+	AuthorityChanges(Vec<SessionKey>),
+}
+
+impl Default for ConcreteLog {
+	fn default() -> Self {
+		unreachable!("shouldn't be called ever")
+	}
+}
+
+impl DigestItem for ConcreteLog {
+	type AuthoritiesChange = Vec<SessionKey>;
+
+	fn as_authorities_change(&self) -> Option<&Self::AuthoritiesChange> {
+		match self {
+			ConcreteLog::AuthorityChanges(ref authorities_change) => Some(authorities_change),
+		}
+	}
+}
+
+impl AuthoritiesChangeDigest<ConcreteLog> for Vec<SessionKey> {
+	type AuthorityId = SessionKey;
+
+	fn new(authorities: Vec<SessionKey>) -> Option<ConcreteLog> {
+		Some(ConcreteLog::AuthorityChanges(authorities))
+	}
+
+	fn authorities(&self) -> &[SessionKey] {
+		&self[..]
+	}
+}
+
 impl_outer_event! {
 	pub enum Event for Concrete {
 		balances, session, staking
@@ -206,7 +243,7 @@ impl_outer_dispatch! {
 /// The address format for describing accounts.
 pub type Address = balances::Address<Concrete>;
 /// Block header type as expected by this runtime.
-pub type Header = generic::Header<BlockNumber, BlakeTwo256, Vec<u8>>;
+pub type Header = generic::Header<BlockNumber, BlakeTwo256, ConcreteLog>;
 /// Block type as expected by this runtime.
 pub type Block = generic::Block<Header, UncheckedExtrinsic>;
 /// BlockId type as expected by this runtime.
diff --git a/demo/runtime/wasm/Cargo.lock b/demo/runtime/wasm/Cargo.lock
index 9bdb08220eccf..f5821a38635d3 100644
--- a/demo/runtime/wasm/Cargo.lock
+++ b/demo/runtime/wasm/Cargo.lock
@@ -652,15 +652,11 @@ dependencies = [
  "substrate-codec-derive 0.1.0",
  "substrate-keyring 0.1.0",
  "substrate-primitives 0.1.0",
- "substrate-runtime-consensus 0.1.0",
  "substrate-runtime-io 0.1.0",
  "substrate-runtime-primitives 0.1.0",
- "substrate-runtime-sandbox 0.1.0",
- "substrate-runtime-session 0.1.0",
  "substrate-runtime-std 0.1.0",
  "substrate-runtime-support 0.1.0",
  "substrate-runtime-system 0.1.0",
- "substrate-runtime-timestamp 0.1.0",
 ]
 
 [[package]]
diff --git a/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.compact.wasm b/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.compact.wasm
index 675b08559e492..2d92622422bcb 100644
Binary files a/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.compact.wasm and b/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.compact.wasm differ
diff --git a/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.wasm b/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.wasm
index 3cbfe39b46b48..361af688f7270 100755
Binary files a/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.wasm and b/demo/runtime/wasm/target/wasm32-unknown-unknown/release/demo_runtime.wasm differ
diff --git a/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.compact.wasm b/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.compact.wasm
index ed66e54638bdb..dea07c96f0b17 100644
Binary files a/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.compact.wasm and b/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.compact.wasm differ
diff --git a/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.wasm b/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.wasm
index 971048d5470b1..c362303349d95 100755
Binary files a/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.wasm and b/substrate/executor/wasm/target/wasm32-unknown-unknown/release/runtime_test.wasm differ
diff --git a/substrate/runtime/consensus/src/lib.rs b/substrate/runtime/consensus/src/lib.rs
index df6d034c8f555..86296e0f8d381 100644
--- a/substrate/runtime/consensus/src/lib.rs
+++ b/substrate/runtime/consensus/src/lib.rs
@@ -39,10 +39,11 @@ extern crate substrate_runtime_system as system;
 extern crate substrate_primitives;
 
 use rstd::prelude::*;
-use runtime_support::{storage, Parameter};
+use runtime_support::{storage, StorageValue, Parameter};
 use runtime_support::dispatch::Result;
 use runtime_support::storage::unhashed::StorageVec;
-use primitives::traits::{MaybeSerializeDebug, MaybeEmpty};
+use primitives::traits::{MaybeSerializeDebug, MaybeEmpty, Convert,
+	Executable, DigestItem, AuthoritiesChangeDigest};
 use primitives::bft::MisbehaviorReport;
 
 #[cfg(any(feature = "std", test))]
@@ -77,6 +78,9 @@ pub trait Trait: system::Trait {
 
 	type SessionKey: Parameter + Default + MaybeSerializeDebug;
 	type OnOfflineValidator: OnOfflineValidator;
+	type ConvertSessionKeyToAuthorityId: Convert<Self::SessionKey,
+		<<system::DigestItemFor<Self> as DigestItem>::AuthoritiesChange as
+			AuthoritiesChangeDigest<system::DigestItemFor<Self>>>::AuthorityId>;
 }
 
 decl_module! {
@@ -96,6 +100,14 @@ decl_module! {
 	}
 }
 
+decl_storage! {
+	trait Store for Module<T: Trait> as Consensus {
+		// Authorities set actual at the block execution start. IsSome only if
+		// the set has been changed.
+		SavedAuthorities get(saved_authorities): default Vec<T::SessionKey>;
+	}
+}
+
 impl<T: Trait> Module<T> {
 	/// Get the current set of authorities. These are the session keys.
 	pub fn authorities() -> Vec<T::SessionKey> {
@@ -149,7 +161,15 @@ impl<T: Trait> Module<T> {
 	///
 	/// Called by `next_session` only.
 	pub fn set_authorities(authorities: &[T::SessionKey]) {
-		AuthorityStorageVec::<T::SessionKey>::set_items(authorities);
+		let previous_authorities = AuthorityStorageVec::<T::SessionKey>::items();
+		if previous_authorities != authorities {
+			let saved_authorities = Self::saved_authorities();
+			if saved_authorities.is_empty() {
+				<SavedAuthorities<T>>::put(previous_authorities);
+			}
+
+			AuthorityStorageVec::<T::SessionKey>::set_items(authorities);
+		}
 	}
 
 	/// Set a single authority by index.
@@ -158,6 +178,15 @@ impl<T: Trait> Module<T> {
 	}
 }
 
+/// Finalization hook for the consensus module.
+impl<T: Trait> Executable for Module<T> {
+	fn execute() {
+		let _saved_authorities = <SavedAuthorities<T>>::take();
+
+		// TODO: call deposit_log for saved_authorities
+	}
+}
+
 #[cfg(any(feature = "std", test))]
 #[derive(Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
diff --git a/substrate/runtime/contract/src/tests.rs b/substrate/runtime/contract/src/tests.rs
index d12b137d41e82..3e4f5f668d61f 100644
--- a/substrate/runtime/contract/src/tests.rs
+++ b/substrate/runtime/contract/src/tests.rs
@@ -32,6 +32,7 @@ pub struct Test;
 impl HasPublicAux for Test {
 	type PublicAux = u64;
 }
+
 impl system::Trait for Test {
 	type PublicAux = <Self as HasPublicAux>::PublicAux;
 	type Index = u64;
diff --git a/substrate/runtime/executive/src/lib.rs b/substrate/runtime/executive/src/lib.rs
index 1d832b73e3fbd..92f50e4a37e9b 100644
--- a/substrate/runtime/executive/src/lib.rs
+++ b/substrate/runtime/executive/src/lib.rs
@@ -231,7 +231,7 @@ mod tests {
 	use runtime_io::with_externalities;
 	use substrate_primitives::{H256, KeccakHasher};
 	use primitives::BuildStorage;
-	use primitives::traits::{HasPublicAux, Identity, Header as HeaderT, BlakeTwo256, AuxLookup};
+	use primitives::traits::{HasPublicAux, Empty, Identity, Header as HeaderT, BlakeTwo256, AuxLookup};
 	use primitives::testing::{Digest, Header, Block};
 
 	struct NullLookup;
@@ -259,6 +259,7 @@ mod tests {
 		const NOTE_OFFLINE_POSITION: u32 = 1;
 		type SessionKey = u64;
 		type OnOfflineValidator = staking::Module<Test>;
+		type ConvertSessionKeyToAuthorityId = Empty;
 	}
 	impl balances::Trait for Test {
 		type Balance = u64;
diff --git a/substrate/runtime/primitives/src/generic.rs b/substrate/runtime/primitives/src/generic.rs
index efcc52fbecb49..6b04412135afe 100644
--- a/substrate/runtime/primitives/src/generic.rs
+++ b/substrate/runtime/primitives/src/generic.rs
@@ -215,7 +215,7 @@ pub struct Digest<Item> {
 }
 
 impl<Item> traits::Digest for Digest<Item> where
-	Item: Member + Default + Codec
+	Item: traits::DigestItem + Default + Codec
 {
 	type Item = Item;
 	fn push(&mut self, item: Self::Item) {
diff --git a/substrate/runtime/primitives/src/testing.rs b/substrate/runtime/primitives/src/testing.rs
index b1dcef2eadbef..69b9101d83cb7 100644
--- a/substrate/runtime/primitives/src/testing.rs
+++ b/substrate/runtime/primitives/src/testing.rs
@@ -36,6 +36,10 @@ impl traits::Digest for Digest {
 	}
 }
 
+impl traits::DigestItem for u64 {
+	type AuthoritiesChange = traits::StubDigestItem;
+}
+
 #[derive(PartialEq, Eq, Clone, Serialize, Deserialize, Debug, Encode, Decode)]
 #[serde(rename_all = "camelCase")]
 #[serde(deny_unknown_fields)]
diff --git a/substrate/runtime/primitives/src/traits.rs b/substrate/runtime/primitives/src/traits.rs
index b64aa678f2867..e67ac0ae7e12e 100644
--- a/substrate/runtime/primitives/src/traits.rs
+++ b/substrate/runtime/primitives/src/traits.rs
@@ -322,13 +322,6 @@ impl<T> MaybeDisplay for T {}
 pub trait Member: Send + Sync + Sized + MaybeSerializeDebug + Eq + PartialEq + Clone + 'static {}
 impl<T: Send + Sync + Sized + MaybeSerializeDebug + Eq + PartialEq + Clone + 'static> Member for T {}
 
-/// Something that acts like a `Digest` - it can have `Log`s `push`ed onto it and these `Log`s are
-/// each `Codec`.
-pub trait Digest {
-	type Item: Member;
-	fn push(&mut self, item: Self::Item);
-}
-
 /// Something which fulfills the abstract idea of a Substrate header. It has types for a `Number`,
 /// a `Hash` and a `Digest`. It provides access to an `extrinsics_root`, `state_root` and
 /// `parent_hash`, as well as a `digest` and a block `number`.
@@ -434,3 +427,46 @@ pub trait Applyable: Sized + Send + Sync {
 	fn sender(&self) -> &Self::AccountId;
 	fn apply(self) -> Result<(), &'static str>;
 }
+
+/// Something that acts like a `Digest` - it can have `Log`s `push`ed onto it and these `Log`s are
+/// each `Codec`.
+pub trait Digest {
+	type Item: DigestItem;
+
+	fn push(&mut self, item: Self::Item);
+}
+
+/// Single digest item.
+pub trait DigestItem: Member {
+	/// Type of authorities change log entry.
+	type AuthoritiesChange: AuthoritiesChangeDigest<Self>; // TODO: = StubDigestItem when associated type defaults are stabilized
+
+	/// Returns Some if the entry is the 'authorities change' log entry.
+	fn as_authorities_change(&self) -> Option<&Self::AuthoritiesChange> {
+		None
+	}
+}
+
+/// Authorities change digest item.
+pub trait AuthoritiesChangeDigest<I: DigestItem> {
+	/// Type of authority Id.
+	type AuthorityId: Member;
+
+	/// Make new authorities change log entry.
+	fn new(authorities: Vec<Self::AuthorityId>) -> Option<I>;
+
+	/// Get new authorities set.
+	fn authorities(&self) -> &[Self::AuthorityId];
+}
+
+/// Empty digest item that is never created and used.
+///
+/// Should be used as a stub for items that are not supported by runtimes.
+pub struct StubDigestItem;
+
+impl<I: DigestItem> AuthoritiesChangeDigest<I> for StubDigestItem {
+	type AuthorityId = ();
+
+	fn new(_authorities: Vec<Self::AuthorityId>) -> Option<I> { None }
+	fn authorities(&self) -> &[Self::AuthorityId] { unreachable!("StubDigestItem is never created") }
+}
diff --git a/substrate/runtime/session/src/lib.rs b/substrate/runtime/session/src/lib.rs
index 75dad0adae76d..27cc3056b35d4 100644
--- a/substrate/runtime/session/src/lib.rs
+++ b/substrate/runtime/session/src/lib.rs
@@ -293,7 +293,7 @@ mod tests {
 	use runtime_io::with_externalities;
 	use substrate_primitives::{H256, KeccakHasher};
 	use primitives::BuildStorage;
-	use primitives::traits::{HasPublicAux, Identity, BlakeTwo256};
+	use primitives::traits::{HasPublicAux, Empty, Identity, BlakeTwo256};
 	use primitives::testing::{Digest, Header};
 
 	#[derive(Clone, Eq, PartialEq)]
@@ -305,6 +305,7 @@ mod tests {
 		const NOTE_OFFLINE_POSITION: u32 = 1;
 		type SessionKey = u64;
 		type OnOfflineValidator = ();
+		type ConvertSessionKeyToAuthorityId = Empty;
 	}
 	impl system::Trait for Test {
 		type PublicAux = <Self as HasPublicAux>::PublicAux;
diff --git a/substrate/runtime/staking/src/mock.rs b/substrate/runtime/staking/src/mock.rs
index 25447c39cbe23..403fb30a60d5f 100644
--- a/substrate/runtime/staking/src/mock.rs
+++ b/substrate/runtime/staking/src/mock.rs
@@ -19,7 +19,7 @@
 #![cfg(test)]
 
 use primitives::BuildStorage;
-use primitives::traits::{HasPublicAux, Identity};
+use primitives::traits::{HasPublicAux, Empty, Identity};
 use primitives::testing::{Digest, Header};
 use substrate_primitives::{H256, KeccakHasher};
 use runtime_io;
@@ -35,6 +35,7 @@ impl consensus::Trait for Test {
 	const NOTE_OFFLINE_POSITION: u32 = 1;
 	type SessionKey = u64;
 	type OnOfflineValidator = ();
+	type ConvertSessionKeyToAuthorityId = Empty;
 }
 impl system::Trait for Test {
 	type PublicAux = <Self as HasPublicAux>::PublicAux;
diff --git a/substrate/runtime/system/src/lib.rs b/substrate/runtime/system/src/lib.rs
index 420192468f2f0..499e686ee5ed9 100644
--- a/substrate/runtime/system/src/lib.rs
+++ b/substrate/runtime/system/src/lib.rs
@@ -86,6 +86,9 @@ pub trait Trait: Eq + Clone {
 	type Event: Parameter + Member;
 }
 
+/// Type of digest item from the Trait.
+pub type DigestItemFor<T> = <<T as Trait>::Digest as traits::Digest>::Item;
+
 decl_module! {
 	pub struct Module<T: Trait>;
 }
diff --git a/substrate/runtime/timestamp/src/lib.rs b/substrate/runtime/timestamp/src/lib.rs
index 80781ab075b64..f0a0ebdfd341f 100644
--- a/substrate/runtime/timestamp/src/lib.rs
+++ b/substrate/runtime/timestamp/src/lib.rs
@@ -143,7 +143,7 @@ mod tests {
 	use runtime_io::with_externalities;
 	use substrate_primitives::H256;
 	use runtime_primitives::BuildStorage;
-	use runtime_primitives::traits::{HasPublicAux, BlakeTwo256};
+	use runtime_primitives::traits::{HasPublicAux, BlakeTwo256, Empty};
 	use runtime_primitives::testing::{Digest, Header};
 
 	#[derive(Clone, Eq, PartialEq)]
@@ -166,6 +166,7 @@ mod tests {
 		const NOTE_OFFLINE_POSITION: u32 = 1;
 		type SessionKey = u64;
 		type OnOfflineValidator = ();
+		type ConvertSessionKeyToAuthorityId = Empty;
 	}
 	impl Trait for Test {
 		const TIMESTAMP_SET_POSITION: u32 = 0;
diff --git a/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.compact.wasm b/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.compact.wasm
index 625a093a6d41b..e428012814621 100644
Binary files a/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.compact.wasm and b/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.compact.wasm differ
diff --git a/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.wasm b/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.wasm
index 874a7a2da0c3f..c1dd3ee207dc5 100755
Binary files a/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.wasm and b/substrate/test-runtime/wasm/target/wasm32-unknown-unknown/release/substrate_test_runtime.wasm differ