diff --git a/substrate/primitives/src/block.rs b/substrate/primitives/src/block.rs
new file mode 100644
index 0000000000..9b3c7beb0d
--- /dev/null
+++ b/substrate/primitives/src/block.rs
@@ -0,0 +1,70 @@
+// Copyright 2017 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Block and header type definitions.
+
+use hash::H256;
+
+/// Hash used to refer to a block hash.
+pub type HeaderHash = H256;
+/// Hash used to refer to proof of block header.
+pub type ProofHash = H256;
+
+/// Unique identifier of a parachain.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
+pub struct ParachainId(u64);
+
+impl From<ParachainId> for u64 {
+	fn from(x: ParachainId) -> Self { x.0 }
+}
+
+impl From<u64> for ParachainId {
+	fn from(x: u64) -> Self { ParachainId(x) }
+}
+
+/// A parachain block proposal.
+#[derive(Debug, PartialEq, Eq)]
+pub struct ParachainProposal {
+	/// The ID of the parachain this is a proposal for.
+	pub parachain: ParachainId,
+	/// Parachain block header bytes.
+	pub header: Vec<u8>,
+	/// Hash of data necessary to prove validity of the header.
+	pub proof_hash: ProofHash,
+}
+
+/// A relay chain block header.
+#[derive(Debug, PartialEq, Eq)]
+pub struct Header {
+	/// Block parent's hash.
+    pub parent_hash: HeaderHash,
+	/// State root after this transition.
+    pub state_root: H256,
+	/// Unix time at which this header was produced.
+    pub timestamp: u64,
+	/// Block number.
+	pub number: u64,
+}
+
+/// A relay chain block body.
+///
+/// Included candidates should be sorted by parachain ID, and without duplicate
+/// IDs.
+#[derive(Debug, PartialEq, Eq)]
+pub struct Body {
+	/// Parachain proposal blocks.
+    pub para_blocks: Vec<ParachainProposal>,
+}
diff --git a/substrate/primitives/src/lib.rs b/substrate/primitives/src/lib.rs
index 74093e6523..d16ba72467 100644
--- a/substrate/primitives/src/lib.rs
+++ b/substrate/primitives/src/lib.rs
@@ -28,8 +28,12 @@ extern crate fixed_hash;
 #[macro_use]
 extern crate uint as uint_crate;
 
-pub mod uint;
+pub mod block;
 pub mod hash;
+pub mod uint;
+
+/// Alias to 160-bit hash when used in the context of an account address.
+pub type Address = hash::H160;
 
 #[cfg(test)]
 mod tests {