diff --git a/src/lib.rs b/src/lib.rs
index 3070723..a9303c6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -53,3 +53,32 @@ pub trait Storage: ReadStorage {
 	/// and might as such do RMW operations at an undesirable performance impact.
 	fn write(&mut self, offset: u32, bytes: &[u8]) -> Result<(), Self::Error>;
 }
+
+/// The size in bytes of a single block read or written by a [`BlockDevice`].
+pub const BLOCK_SIZE: usize = 512;
+
+/// A single block which may be read or written by a [`BlockDevice`].
+///
+/// Also referred to as a sector in some contexts.
+pub type Block = [u8; BLOCK_SIZE];
+
+/// A device which can read and write whole numbers of blocks.
+pub trait BlockDevice {
+	/// The error type returned by methods on this trait.
+	type Error;
+
+	/// Returns the size of the device in blocks.
+	fn block_count(&self) -> Result<usize, Self::Error>;
+
+	/// Reads some number of blocks from the device, starting at `first_block_index`.
+	///
+	/// `first_block_index + blocks.len()` must not be greater than the size returned by
+	/// `block_count`.
+	fn read(&mut self, first_block_index: u64, blocks: &mut [Block]) -> Result<(), Self::Error>;
+
+	/// Writes some number of blocks to the device, starting at `first_block_index`.
+	///
+	/// `first_block_index + blocks.len()` must not be greater than the size returned by
+	/// `block_count`.
+	fn write(&mut self, first_block_index: u64, blocks: &[Block]) -> Result<(), Self::Error>;
+}