diff --git a/circuitpython_typing/__init__.py b/circuitpython_typing/__init__.py
index 5a56cec..a826ae2 100644
--- a/circuitpython_typing/__init__.py
+++ b/circuitpython_typing/__init__.py
@@ -24,6 +24,7 @@
 __all__ = [
     "Alarm",
     "AudioSample",
+    "BlockDevice",
     "ByteStream",
     "FrameBuffer",
     "ReadableBuffer",
@@ -72,6 +73,58 @@ def write(self, buf: ReadableBuffer) -> Optional[int]:
         """Write the bytes in ``buf`` to the stream."""
 
 
+class BlockDevice(Protocol):
+    """Protocol for block device objects to enable a device to support
+    CircuitPython filesystems. Classes which implement this protocol
+    include `storage.VfsFat`.
+    """
+
+    def readblocks(self, block_num: int, buf: bytearray) -> None:
+        """Read aligned, multiples of blocks. Starting at
+        the block given by the index ``block_num``, read blocks
+        from the device into ``buf`` (an array of bytes). The number
+        of blocks to read is given by the length of ``buf``,
+        which will be a multiple of the block size.
+        """
+
+    def writeblocks(self, block_num: int, buf: bytearray) -> None:
+        """Write aligned, multiples of blocks, and require that
+        the blocks that are written to be first erased (if necessary)
+        by this method. Starting at the block given by the index
+        ``block_num``, write blocks from ``buf`` (an array of bytes) to the
+        device. The number of blocks to write is given by the length
+        of ``buf``, which will be a multiple of the block size.
+        """
+
+    def ioctl(self, operation: int, arg: Optional[int] = None) -> Optional[int]:
+        """Control the block device and query its parameters. The operation to
+        perform is given by ``operation`` which is one of the following integers:
+
+        * 1 - initialise the device (``arg`` is unused)
+        * 2 - shutdown the device (``arg`` is unused)
+        * 3 - sync the device (``arg`` is unused)
+        * 4 - get a count of the number of blocks, should return an integer (``arg`` is unused)
+        * 5 - get the number of bytes in a block, should return an integer,
+            or ``None`` in which case the default value of 512 is used (``arg`` is unused)
+        * 6 - erase a block, arg is the block number to erase
+
+        As a minimum ``ioctl(4, ...)`` must be intercepted; for littlefs ``ioctl(6, ...)``
+        must also be intercepted. The need for others is hardware dependent.
+
+        Prior to any call to ``writeblocks(block, ...)`` littlefs issues ``ioctl(6, block)``.
+        This enables a device driver to erase the block prior to a write if the hardware
+        requires it. Alternatively a driver might intercept ``ioctl(6, block)`` and return 0
+        (success). In this case the driver assumes responsibility for detecting the need
+        for erasure.
+
+        Unless otherwise stated ``ioctl(operation, arg)`` can return ``None``. Consequently an
+        implementation can ignore unused values of ``operation``. Where ``operation`` is
+        intercepted, the return value for operations 4 and 5 are as detailed above. Other
+        operations should return 0 on success and non-zero for failure, with the value returned
+        being an ``OSError`` errno code.
+        """
+
+
 # These types may not be in adafruit-blinka, so use the string form instead of a resolved name.
 
 AudioSample: TypeAlias = Union[