diff --git a/src/bytes.rs b/src/bytes.rs
index 870c9eef3..91f9fcda2 100644
--- a/src/bytes.rs
+++ b/src/bytes.rs
@@ -10,6 +10,8 @@ use std::fmt;
 use std::error::Error as StdError;
 use uint::{U128, U256};
 
+pub type Bytes = Vec<u8>;
+
 pub trait BytesConvertable {
 	fn bytes(&self) -> &[u8];
 }
diff --git a/src/hashdb.rs b/src/hashdb.rs
new file mode 100644
index 000000000..b686da78d
--- /dev/null
+++ b/src/hashdb.rs
@@ -0,0 +1,9 @@
+use hash::*;
+use bytes::Bytes;
+
+pub trait HashDB {
+	fn lookup(&self, key: &H256) -> Option<&Bytes>;
+	fn exists(&self, key: &H256) -> bool;
+	fn insert(&mut self, value: &[u8]) -> H256;
+	fn kill(&mut self, key: &H256);
+}
diff --git a/src/lib.rs b/src/lib.rs
index 7c480c02b..a43585e0c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,8 +6,6 @@ extern crate tiny_keccak;
 #[macro_use]
 extern crate log;
 
-pub use std::str::FromStr;
-
 pub mod error;
 pub mod hash;
 pub mod uint;
@@ -16,11 +14,15 @@ pub mod rlp;
 pub mod vector;
 pub mod db;
 pub mod sha3;
+pub mod hashdb;
+pub mod memorydb;
 
 //pub mod network;
 
-pub type Bytes = Vec<u8>;
+pub use std::str::FromStr;
+pub use hash::*;
+pub use sha3::*;
+pub use bytes::*;
+pub use hashdb::*;
+pub use memorydb::*;
 
-#[test]
-fn it_works() {
-}
diff --git a/src/memorydb.rs b/src/memorydb.rs
new file mode 100644
index 000000000..d732b9e5f
--- /dev/null
+++ b/src/memorydb.rs
@@ -0,0 +1,168 @@
+//! Reference-counted memory-based HashDB implementation.
+//! 
+//! # Example
+//! ```rust
+//! extern crate ethcore_util;
+//! use ethcore_util::hashdb::*;
+//! use ethcore_util::memorydb::*;
+//! fn main() {
+//!   let mut m = MemoryDB::new();
+//!   let d = "Hello world!".as_bytes();
+//!
+//!   let k = m.insert(d);
+//!   assert!(m.exists(&k));
+//!   assert_eq!(m.lookup(&k).unwrap(), &d);
+//!
+//!   m.insert(d);
+//!   assert!(m.exists(&k));
+//!
+//!   m.kill(&k);
+//!   assert!(m.exists(&k));
+//!
+//!   m.kill(&k);
+//!   assert!(!m.exists(&k));
+//!
+//!   m.insert(d);
+//!   assert!(m.exists(&k));
+//!   assert_eq!(m.lookup(&k).unwrap(), &d);
+//!
+//!   m.kill(&k);
+//!   assert!(!m.exists(&k));
+//! }
+
+//! ```
+use hash::*;
+use bytes::*;
+use sha3::*;
+use hashdb::*;
+use std::collections::HashMap;
+
+#[derive(Debug,Clone)]
+pub struct MemoryDB {
+	data: HashMap<H256, (Bytes, i32)>,
+}
+
+impl MemoryDB {
+	pub fn new() -> MemoryDB {
+		MemoryDB {
+			data: HashMap::new()
+		}
+	}
+
+	pub fn clear(&mut self) {
+		self.data.clear();
+	}
+
+	pub fn purge(&mut self) {
+		let empties: Vec<_> = self.data.iter().filter(|&(_, &(_, rc))| rc <= 0).map(|(k, _)| k.clone()).collect();
+		for empty in empties { self.data.remove(&empty); }
+	}
+}
+
+impl HashDB for MemoryDB {
+	/// Do a hash dereference and look up a given hash into the bytes that make it up,
+	/// returning None if nothing is found (or if the only entries have 0 or fewer
+	/// references).
+	///
+	/// # Examples
+	/// ```rust
+	/// extern crate ethcore_util;
+	/// use ethcore_util::hashdb::*;
+	/// use ethcore_util::memorydb::*;
+	/// fn main() {
+	///   let mut m = MemoryDB::new();
+	///   let hello_bytes = "Hello world!".as_bytes();
+	///   let hash = m.insert(hello_bytes);
+	///   assert_eq!(m.lookup(&hash).unwrap(), &hello_bytes);
+	/// }
+	/// ```
+	fn lookup(&self, key: &H256) -> Option<&Bytes> {
+		match self.data.get(key) {
+			Some(&(ref d, rc)) if rc > 0 => Some(d),
+			_ => None
+		}
+	}
+
+	/// Check for the existance of a hash-key.
+	///
+	/// # Examples
+	/// ```rust
+	/// extern crate ethcore_util;
+	/// use ethcore_util::hashdb::*;
+	/// use ethcore_util::memorydb::*;
+	/// use ethcore_util::sha3::*;
+	/// fn main() {
+	///   let mut m = MemoryDB::new();
+	///   let hello_bytes = "Hello world!".as_bytes();
+	///   assert!(!m.exists(&hello_bytes.sha3()));
+	///   let key = m.insert(hello_bytes);
+	///   assert!(m.exists(&key));
+	///   m.kill(&key);
+	///   assert!(!m.exists(&key));
+	/// }
+	/// ```
+	fn exists(&self, key: &H256) -> bool {
+		match self.data.get(key) {
+			Some(&(_, x)) if x > 0 => true,
+			_ => false
+		}
+	}
+
+	/// Insert a datum item into the DB and return the datum's hash for a later lookup. Insertions
+	/// are counted and the equivalent number of `kill()`s must be performed before the data
+	/// is considered dead.
+	///
+	/// # Examples
+	/// ```rust
+	/// extern crate ethcore_util;
+	/// use ethcore_util::hashdb::*;
+	/// use ethcore_util::memorydb::*;
+	/// use ethcore_util::hash::*;
+	/// fn main() {
+	///   let mut m = MemoryDB::new();
+	///   let key = m.insert("Hello world!".as_bytes());
+	///   assert!(m.exists(&key));
+	/// }
+	/// ```
+	fn insert(&mut self, value: &[u8]) -> H256 {
+		let key = value.sha3();
+		if match self.data.get_mut(&key) {
+			Some(&mut (ref mut old_value, ref mut rc @ 0)) => { *old_value = From::from(value.bytes()); *rc = 1; false },
+			Some(&mut (_, ref mut x)) => { *x += 1; false } ,
+			None => true,
+		}{	// ... None falls through into...
+			self.data.insert(key, (From::from(value.bytes()), 1));
+		}
+		key
+	}
+	/// Remove a datum previously inserted. Insertions can be "owed" such that the same number of inserts may
+	/// happen without the data being eventually being inserted into the DB.
+	///
+	/// # Examples
+	/// ```rust
+	/// extern crate ethcore_util;
+	/// use ethcore_util::hashdb::*;
+	/// use ethcore_util::memorydb::*;
+	/// use ethcore_util::sha3::*;
+	/// fn main() {
+	///   let mut m = MemoryDB::new();
+	///   let d = "Hello world!".as_bytes();
+	///   let key = &d.sha3();
+	///   m.kill(key);	// OK - we now owe an insertion.
+	///   assert!(!m.exists(key));
+	///   m.insert(d);	// OK - now it's "empty" again.
+	///   assert!(!m.exists(key));
+	///   m.insert(d);	// OK - now we've
+	///   assert_eq!(m.lookup(key).unwrap(), &d);
+	/// }
+	/// ```
+	fn kill(&mut self, key: &H256) {
+		if match self.data.get_mut(key) {
+			Some(&mut (_, ref mut x)) => { *x -= 1; false }
+			None => true
+		}{	// ... None falls through into...
+			self.data.insert(*key, (Bytes::new(), -1));
+		}
+	}
+}
+
diff --git a/src/sha3.rs b/src/sha3.rs
index e2866d288..e4ba1b007 100644
--- a/src/sha3.rs
+++ b/src/sha3.rs
@@ -3,7 +3,7 @@ use tiny_keccak::keccak_256;
 use bytes::BytesConvertable;
 use hash::H256;
 
-trait Hashable {
+pub trait Hashable {
 	fn sha3(&self) -> H256;
 }