diff --git a/substrate/native-runtime/std/src/lib.rs b/substrate/native-runtime/std/src/lib.rs
index 24a6693375..be67b81bab 100644
--- a/substrate/native-runtime/std/src/lib.rs
+++ b/substrate/native-runtime/std/src/lib.rs
@@ -93,9 +93,9 @@ pub fn chain_id() -> u64 {
 }
 
 /// "Commit" all existing operations and get the resultant storage root.
-pub fn commit() -> [u8; 32] {
+pub fn storage_root() -> [u8; 32] {
 	ext::with(|ext|
-		ext.commit()
+		ext.storage_root()
 	).unwrap_or([0u8; 32])
 }
 
diff --git a/substrate/state_machine/src/backend.rs b/substrate/state_machine/src/backend.rs
index ee6b0bbe80..9f07f2d98c 100644
--- a/substrate/state_machine/src/backend.rs
+++ b/substrate/state_machine/src/backend.rs
@@ -40,6 +40,9 @@ pub trait Backend {
 	/// Commit updates to the backend and get new state.
 	fn commit<I>(&mut self, changes: I) -> Committed
 		where I: IntoIterator<Item=Update>;
+
+	/// Get all key/value pairs into a Vec.
+	fn pairs(&self) -> Vec<(&[u8], &[u8])>;
 }
 
 /// Error impossible.
@@ -87,6 +90,10 @@ impl Backend for InMemory {
 			storage_tree_root,
 		}
 	}
+
+	fn pairs(&self) -> Vec<(&[u8], &[u8])> {
+		self.inner.storage.iter().map(|(k, v)| (&k[..], &v[..])).collect()
+	}
 }
 
 // TODO: DB-based backend
diff --git a/substrate/state_machine/src/ext.rs b/substrate/state_machine/src/ext.rs
index 02901c48d5..1beca17fa6 100644
--- a/substrate/state_machine/src/ext.rs
+++ b/substrate/state_machine/src/ext.rs
@@ -17,7 +17,7 @@
 //! Conrete externalities implementation.
 
 use std::{error, fmt};
-
+use triehash::trie_root;
 use backend::Backend;
 use {Externalities, ExternalitiesError, OverlayedChanges};
 
@@ -76,7 +76,14 @@ impl<'a, B: 'a> Externalities for Ext<'a, B>
 		42
 	}
 
-	fn commit(&self) -> [u8; 32] {
-		unimplemented!();
+	fn storage_root(&self) -> [u8; 32] {
+		let mut all_pairs = self.backend.pairs();
+		all_pairs.extend(
+			self.overlay.committed.storage.iter()
+				.chain(self.overlay.prospective.storage.iter())
+				.map(|(k, v)| (&k[..], &v[..]))
+		);
+
+		trie_root(all_pairs.into_iter().map(|(k, v)| (k.to_vec(), v.to_vec()))).0
 	}
 }
diff --git a/substrate/state_machine/src/lib.rs b/substrate/state_machine/src/lib.rs
index 61b1fafd37..e1e674a419 100644
--- a/substrate/state_machine/src/lib.rs
+++ b/substrate/state_machine/src/lib.rs
@@ -147,7 +147,7 @@ pub trait Externalities {
 	fn chain_id(&self) -> u64;
 
 	/// Get the trie root of the current storage map.
-	fn commit(&self) -> [u8; 32];
+	fn storage_root(&self) -> [u8; 32];
 
 	/// Get the current set of authorities from storage.
 	fn authorities(&self) -> Result<Vec<&[u8]>, ExternalitiesError> {
diff --git a/substrate/state_machine/src/testing.rs b/substrate/state_machine/src/testing.rs
index 74cfbb9dd5..1c9c4dfcf8 100644
--- a/substrate/state_machine/src/testing.rs
+++ b/substrate/state_machine/src/testing.rs
@@ -23,11 +23,13 @@ use triehash::trie_root;
 /// Simple HashMap based Externalities impl.
 #[derive(Debug, Default)]
 pub struct TestExternalities {
+	/// The storage.
 	pub storage: HashMap<Vec<u8>, Vec<u8>>,
 }
 
 impl TestExternalities {
-	fn new() -> Self {
+	/// Create a new instance with empty storage.
+	pub fn new() -> Self {
 		TestExternalities {
 			storage: HashMap::new(),
 		}
@@ -45,7 +47,7 @@ impl Externalities for TestExternalities {
 
 	fn chain_id(&self) -> u64 { 42 }
 
-	fn commit(&self) -> [u8; 32] {
+	fn storage_root(&self) -> [u8; 32] {
 		trie_root(self.storage.clone()).0
 	}
 }
@@ -61,6 +63,6 @@ mod tests {
 		ext.set_storage(b"dog".to_vec(), b"puppy".to_vec());
 		ext.set_storage(b"dogglesworth".to_vec(), b"cat".to_vec());
 		const ROOT: [u8; 32] = hex!("8aad789dff2f538bca5d8ea56e8abe10f4c7ba3a5dea95fea4cd6e7c3a1168d3");
-		assert_eq!(ext.commit(), ROOT);
+		assert_eq!(ext.storage_root(), ROOT);
 	}
 }