diff --git a/USAGE.md b/USAGE.md
index 8a83c14..99339c4 100644
--- a/USAGE.md
+++ b/USAGE.md
@@ -10,8 +10,10 @@
     * [`bitser.includeMetatables`](#includeMetatables)
     * [`bitser.register`](#register)
     * [`bitser.registerClass`](#registerclass)
+    * [`bitser.registerUserdata`](#registerUserdata)
     * [`bitser.unregister`](#unregister)
     * [`bitser.unregisterClass`](#unregisterclass)
+    * [`bitser.unregisterUserdata`](#unregisterUserdata)
     * [`bitser.reserveBuffer`](#reservebuffer)
     * [`bitser.clearBuffer`](#clearbuffer)
 
@@ -202,6 +204,55 @@ Returns the registered class as a convenience.
 
 See also: [`bitser.unregisterClass`](#unregisterclass).
 
+## registerUserdata
+
+```lua
+bitser.registerUserdata(id, matcher, serializer, deserializer)
+```
+
+This is an advanved-users-only feature, allowing you to register a custom userdata handler.
+This makes it possible for bitser to serialize and deserialize userdata with your own callbacks.
+
+- The `id` parameter has to be a number from 0 to 255 and unique, and identifies your userdata handler.
+It must be unique per userdata "type".
+
+- The `matcher` function takes the userdata value as parameter, and must return a boolean: whether the
+current userdata value is (matches) the one you registered the handler for. This could be done via metatable
+lookup, for instance, or any other way that your userdata provides.
+
+- The `serializer` function takes the value (your userdata) and an `env` table as parameters.
+  The `env` table provides access to common internal functions useful for (de)serializing, and the `seen` data.
+
+- The `deserializer` function takes an `env` table as parameter.
+  The `env` table provides access to common internal functions useful for (de)serializing, and the `seen` data.
+
+Full usage example with a `ByteArray` class as userdata, which under the hood, stores bytes sequentially:  
+(The internal format would be `[len][data...]`)
+```lua
+bitser.registerUserdata(1,
+  function(value)
+    return getmetatable(value) == ByteArray
+  end,
+  function(value, env)
+    local len = #value
+    env.serialize_value(len, env.seen)
+    env.Buffer_write_raw(ffi.string(ffi.cast("uint8_t*", value:getDataPtr()), len), len)
+  end,
+  function(env)
+    local len = env.deserialize_value(env.seen)
+    local ba = ByteArray(len)
+    env.Buffer_read_raw(ffi.cast("uint8_t*", ba:getDataPtr()), len)
+    return ba
+end)
+
+local test = ByteArray({ 0x01, 0x02, 0x00, 0x04 })
+local test2 = bitser.loads(bitser.dumps(test))
+
+print(test:tostring("base64") == test2:tostring("base64")) -- true
+```
+
+See also: [`bitser.unregisterUserdata`](#unregisterUserdata).
+
 ## unregister
 
 ```lua
@@ -223,6 +274,16 @@ which is useful in a context where you don't have a reference to the class you w
 
 See also: [`bitser.registerClass`](#registerclass).
 
+## unregisterUserdata
+
+```lua
+bitser.unregisterUserdata(id)
+```
+
+Deregisters the previously registered userdata with the id `id`.
+
+See also: [`bitser.registerUserdata`](#registerUserdata).
+
 ## reserveBuffer
 
 ```lua
diff --git a/bitser.lua b/bitser.lua
index b76a9d7..9387762 100644
--- a/bitser.lua
+++ b/bitser.lua
@@ -32,6 +32,7 @@ local writable_buf = nil
 local writable_buf_size = nil
 local includeMetatables = true -- togglable with bitser.includeMetatables(false)
 local SEEN_LEN = {}
+local shared_env = {} -- for userdata callbacks
 
 local function Buffer_prereserve(min_size)
 	if buf_size < min_size then
@@ -139,6 +140,7 @@ end
 
 local resource_registry = {}
 local resource_name_registry = {}
+local userdata_registry = {}
 local class_registry = {}
 local class_name_registry = {}
 local classkey_registry = {}
@@ -243,7 +245,20 @@ local function write_cdata(value, seen)
 	Buffer_write_raw(ffi.typeof('$[1]', ty)(value), len)
 end
 
-local types = {number = write_number, string = write_string, table = write_table, boolean = write_boolean, ["nil"] = write_nil, cdata = write_cdata}
+local function write_userdata(value, seen)
+	for ud_id, ud_handler in pairs(userdata_registry) do
+		if ud_handler.match(value) then
+			Buffer_write_byte(254)
+			Buffer_write_byte(ud_id)
+			shared_env.seen = seen
+			ud_handler.serialize(value, shared_env)
+			return
+		end
+	end
+	error("cannot serialize this userdata")
+end
+
+local types = {number = write_number, string = write_string, table = write_table, boolean = write_boolean, ["nil"] = write_nil, cdata = write_cdata, userdata = write_userdata}
 
 serialize_value = function(value, seen)
 	if seen[value] then
@@ -259,7 +274,7 @@ serialize_value = function(value, seen)
 		return
 	end
 	local t = type(value)
-	if t ~= 'number' and t ~= 'boolean' and t ~= 'nil' and t ~= 'cdata' then
+	if t ~= 'number' and t ~= 'boolean' and t ~= 'nil' and t ~= 'cdata' and t ~= 'userdata' then
 		seen[value] = seen[SEEN_LEN]
 		seen[SEEN_LEN] = seen[SEEN_LEN] + 1
 	end
@@ -388,11 +403,25 @@ local function deserialize_value(seen)
 		local read_into = ffi.typeof('$[1]', ctype)()
 		Buffer_read_raw(read_into, len)
 		return ctype(read_into[0])
+	elseif t == 254 then
+		--userdata
+		local ud_id = Buffer_read_byte()
+		local ud_handler = userdata_registry[ud_id]
+		if ud_handler then
+			shared_env.seen = seen
+			return ud_handler.deserialize(shared_env)
+		end
+		error("unsupported serialized userdata id " .. ud_id)
 	else
 		error("unsupported serialized type " .. t)
 	end
 end
 
+shared_env.serialize_value = serialize_value
+shared_env.deserialize_value = deserialize_value
+shared_env.Buffer_write_raw = Buffer_write_raw
+shared_env.Buffer_read_raw = Buffer_read_raw
+
 local function deserialize_MiddleClass(instance, class)
 	return setmetatable(instance, class.__instanceDict)
 end
@@ -493,4 +522,10 @@ end, unregisterClass = function(name)
 	classkey_registry[name] = nil
 	class_deserialize_registry[name] = nil
 	class_registry[name] = nil
+end, registerUserdata = function(id, matcher, serializer, deserializer)
+	assert(type(id) == "number" and id >= 0 and id <= 255, "registerUserdata: id must be a number between 0 and 255")
+	assert(not userdata_registry[id], "registerUserdata: id " .. id .. " already registered")
+	userdata_registry[id] = { match = matcher, serialize = serializer, deserialize = deserializer}
+end, unregisterUserdata = function(id)
+	userdata_registry[id] = nil
 end, reserveBuffer = Buffer_prereserve, clearBuffer = Buffer_clear, version = VERSION}
diff --git a/spec/bitser_spec.lua b/spec/bitser_spec.lua
index c292b99..b0c0c95 100644
--- a/spec/bitser_spec.lua
+++ b/spec/bitser_spec.lua
@@ -274,7 +274,6 @@ describe("bitser", function()
 		assert.has_error(function() bitser.registerClass('Horse', {mane = 'majestic'}) end, "no deserializer given for unsupported class library")
 	end)
 	it("cannot deserialize values from unassigned type bytes", function()
-		assert.has_error(function() bitser.loads("\254") end, "unsupported serialized type 254")
 		assert.has_error(function() bitser.loads("\255") end, "unsupported serialized type 255")
 	end)
 	it("can load from raw data", function()