diff --git a/CHANGELOG.md b/CHANGELOG.md
index 493ecf2b60..b75bbef459 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 -## V 0.2.5
 -*Not yet released, changelog is not full*
-- Introduce `isize` and `usize` types, deprecate `size_t` in favor of `usize`
+- Introduce `isize` and `usize` types, deprecate `size_t` in favor of `usize`.
+- Add `datatypes` and `datatypes.fsm` modules.
 
 -## V 0.2.4
 -*Not yet released, changelog is not full*
diff --git a/vlib/datatypes/fsm/README.md b/vlib/datatypes/fsm/README.md
new file mode 100644
index 0000000000..8b62a99cd0
--- /dev/null
+++ b/vlib/datatypes/fsm/README.md
@@ -0,0 +1,21 @@
+# fsm
+
+This module implements a Finite State Machine (FSM).
+The FSM is composed of states and transitions between them.
+These need to be specified by the client.
+
+## Usage
+
+Have a look at `fsm_test.v` for usage examples.
+
+On each `run()`, all the possible transitions from the current state are evaluated.
+The first transition for the current state, whose condition evaluates to true is
+taken (the condition is specified by a transition callback function). 
+
+In a successfull transition, the current state changes to the new one.
+When that happens:
+* the client-specified `on_exit()` handler from the current state is called.
+* the client-specified `on_entry()` handler of the new state is called.
+
+After all transitions are checked, and thus the state is changed, the client-specified
+`on_run()` handler of the now current state is called.
diff --git a/vlib/datatypes/fsm/fsm.v b/vlib/datatypes/fsm/fsm.v
new file mode 100644
index 0000000000..3d8f309670
--- /dev/null
+++ b/vlib/datatypes/fsm/fsm.v
@@ -0,0 +1,87 @@
+module fsm
+
+pub type EventHandlerFn = fn (receiver voidptr, from string, to string)
+
+pub type ConditionFn = fn (receiver voidptr, from string, to string) bool
+
+struct State {
+mut:
+	entry_handler EventHandlerFn
+	run_handler   EventHandlerFn
+	exit_handler  EventHandlerFn
+}
+
+struct Transition {
+mut:
+	to                string
+	condition_handler ConditionFn = voidptr(0)
+}
+
+pub struct StateMachine {
+mut:
+	states        map[string]State
+	transitions   map[string][]Transition
+	current_state string
+}
+
+pub fn new() StateMachine {
+	return StateMachine{}
+}
+
+pub fn (mut s StateMachine) set_state(name string) ? {
+	if name in s.states {
+		s.current_state = name
+	}
+	return error('unknown state: $name')
+}
+
+pub fn (mut s StateMachine) get_state() string {
+	return s.current_state
+}
+
+pub fn (mut s StateMachine) add_state(name string, entry EventHandlerFn, run EventHandlerFn, exit EventHandlerFn) {
+	s.states[name] = State{
+		entry_handler: entry
+		run_handler: run
+		exit_handler: exit
+	}
+	if s.states.len == 1 {
+		s.current_state = name
+	}
+}
+
+pub fn (mut s StateMachine) add_transition(from string, to string, condition_handler ConditionFn) {
+	t := Transition{
+		to: to
+		condition_handler: condition_handler
+	}
+	if from in s.transitions {
+		s.transitions[from] << t
+		return
+	}
+	s.transitions[from] = [t]
+}
+
+pub fn (mut s StateMachine) run(receiver voidptr) ? {
+	from_state := s.current_state
+	mut to_state := s.current_state
+	if transitions := s.transitions[s.current_state] {
+		for transition in transitions {
+			if transition.condition_handler(receiver, from_state, transition.to) {
+				s.change_state(receiver, transition.to)
+				to_state = transition.to
+				break
+			}
+		}
+	} else {
+		s.states[s.current_state].run_handler(receiver, from_state, to_state)
+		return error('no more transitions')
+	}
+	s.states[s.current_state].run_handler(receiver, from_state, to_state)
+}
+
+fn (mut s StateMachine) change_state(receiver voidptr, newstate string) {
+	s.states[s.current_state].exit_handler(receiver, s.current_state, newstate)
+	s.states[newstate].entry_handler(receiver, s.current_state, newstate)
+	s.current_state = newstate
+}
diff --git a/vlib/datatypes/fsm/fsm_test.v b/vlib/datatypes/fsm/fsm_test.v
new file mode 100644
index 0000000000..ef4136ea36
--- /dev/null
+++ b/vlib/datatypes/fsm/fsm_test.v
@@ -0,0 +1,88 @@
+import datatypes.fsm
+
+struct MyReceiver {
+mut:
+	data []string
+}
+
+fn default_setup() (MyReceiver, fsm.StateMachine) {
+	mut receiver := MyReceiver{}
+	mut s := fsm.new()
+	s.add_state('A', on_state_entry, on_state_run, on_state_exit)
+	s.add_state('B', on_state_entry, on_state_run, on_state_exit)
+	s.add_transition('A', 'B', condition_transition)
+	return receiver, s
+}
+
+fn test_statemachine_number_of_callbacks_correct_when_single_transition() ? {
+	mut receiver, mut s := default_setup()
+
+	s.run(receiver) ?
+
+	assert receiver.data.len == 3
+}
+
+fn test_statemachine_sequence_works_when_typical() ? {
+	mut receiver, mut s := default_setup()
+
+	s.run(receiver) ?
+
+	assert receiver.data[0] == 'on_state_exit: A -> B'
+	assert receiver.data[1] == 'on_state_entry: A -> B'
+	assert receiver.data[2] == 'on_state_run: A -> B'
+}
+
+fn test_statemachine_works_when_final_state() ? {
+	mut receiver, mut s := default_setup()
+
+	// current state `A`, with a possible transition to `B`:
+	s.run(receiver) ? // run should not error here
+
+	// Note: run will now return error, because for state `B`,
+	// there are no more transitions:
+	s.run(receiver) or { assert true }
+	s.run(receiver) or { assert true }
+
+	assert receiver.data.len == 5
+	assert receiver.data[2] == 'on_state_run: A -> B'
+	assert receiver.data[3] == 'on_state_run: B -> B'
+	assert receiver.data[4] == 'on_state_run: B -> B'
+}
+
+fn test_simple_loop() ? {
+	mut receiver, mut s := default_setup()
+
+	// Add a transition back to `A` too:
+	s.add_transition('B', 'A', condition_transition)
+
+	// Run the FSM for a while.
+	// It will loop forever between `A` -> `B` -> `A` -> `B` ...
+	for _ in 0 .. 100 {
+		s.run(receiver) or { assert false }
+	}
+	assert receiver.data[1] == 'on_state_entry: A -> B'
+	assert receiver.data[4] == 'on_state_entry: B -> A'
+	assert receiver.data[7] == 'on_state_entry: A -> B'
+	assert receiver.data[10] == 'on_state_entry: B -> A'
+}
+
+// Helper functions
+
+fn on_state_entry(mut receiver MyReceiver, from string, to string) {
+	receiver.data << 'on_state_entry: ' + from + ' -> ' + to
+}
+
+fn on_state_run(mut receiver MyReceiver, from string, to string) {
+	receiver.data << 'on_state_run: ' + from + ' -> ' + to
+}
+
+fn on_state_exit(mut receiver MyReceiver, from string, to string) {
+	receiver.data << 'on_state_exit: ' + from + ' -> ' + to
+}
+
+fn condition_transition(receiver &MyReceiver, from string, to string) bool {
+	// The condition callback is a way to provide input to the FSM
+	// It can return true or false, based on external events/state.
+	// For these tests however, that is not used, and it simply always returns true.
+	return true
+}