diff --git a/safeeyes/tests/test_core.py b/safeeyes/tests/test_core.py
new file mode 100644
index 0000000..c77e264
--- /dev/null
+++ b/safeeyes/tests/test_core.py
@@ -0,0 +1,345 @@
+from collections import deque
+import datetime
+import logging
+import pytest
+
+from safeeyes import core
+from safeeyes import model
+
+import threading
+from time import sleep
+
+from unittest import mock
+
+class TestSafeEyesCore:
+    @pytest.fixture(autouse=True)
+    def set_time(self, time_machine):
+        time_machine.move_to(datetime.datetime.fromisoformat("2024-08-25T13:00"))
+
+    @pytest.fixture(autouse=True)
+    def monkeypatch_translations(self, monkeypatch):
+        monkeypatch.setattr(core, "_", lambda message: "translated!: " + message, raising=False)
+        monkeypatch.setattr(model, "_", lambda message: "translated!: " + message, raising=False)
+
+    @pytest.fixture
+    def sequential_threading(self, monkeypatch):
+        # executes instantly
+        # TODO: separate thread?
+        monkeypatch.setattr(
+            core.utility,
+            "execute_main_thread",
+            lambda target_function, *args, **kwargs: target_function(*args, **kwargs)
+        )
+
+
+        class Handle:
+            thread = None
+            task_queue = deque()
+            running = True
+            condvar_in = threading.Condition()
+            condvar_out = threading.Condition()
+
+            def background_thread(self):
+                while True:
+                    with self.condvar_in:
+                        success = self.condvar_in.wait(1)
+                        if not success:
+                            raise Exception("thread timed out")
+
+                    if not self.running:
+                        logging.debug(f"background task shutdown")
+                        break
+
+                    logging.debug(f"background task woken up")
+
+                    if self.task_queue:
+                        (target_function, kwargs) = self.task_queue.popleft()
+                        logging.debug(f"thread started {target_function}")
+                        target_function(**kwargs)
+                        logging.debug(f"thread finished {target_function}")
+
+                    with self.condvar_out:
+                        self.condvar_out.notify()
+
+            def sleep(self, time):
+                if self.thread is threading.current_thread():
+                    with self.condvar_out:
+                        self.condvar_out.notify()
+                    with self.condvar_in:
+                        success = self.condvar_in.wait(1)
+                        if not success:
+                            raise Exception("thread timed out")
+
+            def utility_start_thread(self, target_function, **kwargs):
+                self.task_queue.append((target_function, kwargs))
+
+                if self.thread is None:
+                    self.thread = threading.Thread(target=self.background_thread, name="WorkThread", daemon=False, kwargs=kwargs)
+                    self.thread.start()
+
+            def next(self):
+                assert self.thread
+
+                with self.condvar_in:
+                    self.condvar_in.notify()
+
+            def wait(self):
+                # wait until done:
+                with self.condvar_out:
+                    success = self.condvar_out.wait(1)
+                    if not success:
+                        raise Exception("thread timed out")
+
+            def stop(self):
+                self.running = False
+                with self.condvar_in:
+                    self.condvar_in.notify()
+
+                if self.thread:
+                    self.thread.join(1)
+
+        handle = Handle()
+
+        monkeypatch.setattr(
+            core.utility,
+            "start_thread",
+            handle.utility_start_thread
+        )
+
+        monkeypatch.setattr(
+            core.time,
+            "sleep",
+            lambda time: handle.sleep(time)
+        )
+
+        yield handle
+
+        handle.stop()
+
+    def test_create_empty(self):
+        context = {}
+        config = {
+            "short_breaks": [],
+            "long_breaks": [],
+            "short_break_interval": 15,
+            "long_break_interval": 75,
+            "long_break_duration": 60,
+            "short_break_duration": 15,
+            "random_order": False,
+            "postpone_duration": 5,
+        }
+        safe_eyes_core = core.SafeEyesCore(context)
+        safe_eyes_core.initialize(config)
+
+
+    def test_start_empty(self, sequential_threading):
+
+        context = {}
+        config = {
+            "short_breaks": [],
+            "long_breaks": [],
+            "short_break_interval": 15,
+            "long_break_interval": 75,
+            "long_break_duration": 60,
+            "short_break_duration": 15,
+            "random_order": False,
+            "postpone_duration": 5,
+        }
+        on_update_next_break = mock.Mock()
+        safe_eyes_core = core.SafeEyesCore(context)
+        safe_eyes_core.on_update_next_break += mock
+
+        safe_eyes_core.initialize(config)
+
+        safe_eyes_core.start()
+        safe_eyes_core.stop()
+
+        on_update_next_break.assert_not_called()
+
+
+    def test_start(self, sequential_threading, time_machine):
+        context = {
+            "session": {},
+        }
+        config = {
+            "short_breaks": [
+                {"name": "break 1"},
+                {"name": "break 2"},
+                {"name": "break 3"},
+                {"name": "break 4"},
+            ],
+            "long_breaks": [
+                {"name": "long break 1"},
+                {"name": "long break 2"},
+                {"name": "long break 3"},
+            ],
+            "short_break_interval": 15,
+            "long_break_interval": 75,
+            "long_break_duration": 60,
+            "short_break_duration": 15,
+            "random_order": False,
+            "postpone_duration": 5,
+        }
+        on_update_next_break = mock.Mock()
+        safe_eyes_core = core.SafeEyesCore(context)
+        safe_eyes_core.on_update_next_break += on_update_next_break
+
+        safe_eyes_core.initialize(config)
+
+        safe_eyes_core.start()
+
+        # start __scheduler_job
+        sequential_threading.next()
+        # FIXME: sleep is needed so code reaches the Condition
+        sleep(0.1)
+        assert context['state'] == model.State.WAITING
+
+        on_update_next_break.assert_called_once()
+        assert isinstance(on_update_next_break.call_args[0][0], model.Break)
+        assert on_update_next_break.call_args[0][0].name == "translated!: break 1"
+        on_update_next_break.reset_mock()
+
+        with safe_eyes_core.lock:
+            time_machine.shift(delta=datetime.timedelta(minutes=15))
+
+            with safe_eyes_core.waiting_condition:
+                logging.debug("notify")
+                safe_eyes_core.waiting_condition.notify_all()
+
+        logging.debug("wait for end of __scheduler_job")
+        sequential_threading.wait()
+        logging.debug("done waiting for end of __scheduler_job")
+
+
+        safe_eyes_core.stop()
+        assert context['state'] == model.State.STOPPED
+
+        logging.debug("done")
+
+
+    def test_actual(self, sequential_threading, time_machine):
+        context = {
+            "session": {},
+        }
+        config = {
+            "short_breaks": [
+                {"name": "break 1"},
+                {"name": "break 2"},
+                {"name": "break 3"},
+                {"name": "break 4"},
+            ],
+            "long_breaks": [
+                {"name": "long break 1"},
+                {"name": "long break 2"},
+                {"name": "long break 3"},
+            ],
+            "short_break_interval": 15,
+            "long_break_interval": 75,
+            "long_break_duration": 60,
+            "short_break_duration": 15,
+            "pre_break_warning_time": 10,
+            "random_order": False,
+            "postpone_duration": 5,
+        }
+        on_update_next_break = mock.Mock()
+        on_pre_break = mock.Mock(return_value=True)
+        on_start_break = mock.Mock(return_value=True)
+        start_break = mock.Mock()
+        on_count_down = mock.Mock()
+        safe_eyes_core = core.SafeEyesCore(context)
+        safe_eyes_core.on_update_next_break += on_update_next_break
+        safe_eyes_core.on_pre_break += on_pre_break
+        safe_eyes_core.on_start_break += on_start_break
+        safe_eyes_core.start_break += start_break
+        safe_eyes_core.on_count_down += on_count_down
+
+        safe_eyes_core.initialize(config)
+
+        safe_eyes_core.start()
+
+        # start __scheduler_job
+        sequential_threading.next()
+        # FIXME: sleep is needed so code reaches the Condition
+        sleep(0.1)
+        assert context['state'] == model.State.WAITING
+
+        on_update_next_break.assert_called_once()
+        assert isinstance(on_update_next_break.call_args[0][0], model.Break)
+        assert on_update_next_break.call_args[0][0].name == "translated!: break 1"
+        on_update_next_break.reset_mock()
+
+        with safe_eyes_core.lock:
+            time_machine.shift(delta=datetime.timedelta(minutes=15))
+
+            with safe_eyes_core.waiting_condition:
+                logging.debug("notify")
+                safe_eyes_core.waiting_condition.notify_all()
+
+        logging.debug("wait for end of __scheduler_job")
+        sequential_threading.wait()
+        logging.debug("done waiting for end of __scheduler_job")
+
+        assert context['state'] == model.State.PRE_BREAK
+
+        on_pre_break.assert_called_once()
+        assert isinstance(on_pre_break.call_args[0][0], model.Break)
+        assert on_pre_break.call_args[0][0].name == "translated!: break 1"
+        on_pre_break.reset_mock()
+
+        # start __wait_until_prepare
+        sequential_threading.next()
+
+        # FIXME: sleep is needed so code reaches the Condition
+        sleep(0.1)
+        with safe_eyes_core.lock:
+            time_machine.shift(delta=datetime.timedelta(seconds=10))
+
+            with safe_eyes_core.waiting_condition:
+                logging.debug("notify")
+                safe_eyes_core.waiting_condition.notify_all()
+
+        logging.debug("wait for end of __wait_until_prepare")
+        sequential_threading.wait()
+        logging.debug("done waiting for end of __wait_until_prepare")
+
+        # start __start_break
+        sequential_threading.next()
+        sequential_threading.wait()
+
+        # first sleep in __start_break
+        sequential_threading.next()
+
+        on_start_break.assert_called_once()
+        assert isinstance(on_start_break.call_args[0][0], model.Break)
+        assert on_start_break.call_args[0][0].name == "translated!: break 1"
+        on_start_break.reset_mock()
+
+        start_break.assert_called_once()
+        assert isinstance(start_break.call_args[0][0], model.Break)
+        assert start_break.call_args[0][0].name == "translated!: break 1"
+        start_break.reset_mock()
+
+        assert context['state'] == model.State.BREAK
+
+        # continue sleep in __start_break
+        for i in range(config["short_break_duration"] - 1):
+            sequential_threading.wait()
+            sequential_threading.next()
+
+        logging.debug("wait for end of __start_break")
+        sequential_threading.wait()
+        logging.debug("done waiting for end of __start_break")
+
+        on_count_down.assert_called()
+        assert on_count_down.call_count == 15
+        on_count_down.reset_mock()
+
+        assert context['state'] == model.State.BREAK
+
+        safe_eyes_core.stop()
+
+        on_update_next_break.assert_not_called()
+        on_pre_break.assert_not_called()
+        on_start_break.assert_not_called()
+        start_break.assert_not_called()
+        assert context['state'] == model.State.STOPPED