diff --git a/README.md b/README.md
index 6854f37..7a8a38b 100644
--- a/README.md
+++ b/README.md
@@ -142,14 +142,12 @@ ProductId = NewType('ProductId', int)
 BasketId = NewType('BasketId', int)
 
 
-@dataclass(kw_only=True)
 class Product(Entity):
   product_id: ProductId
   name: ProductName
   amount: float = 0
 
 
-@dataclass(kw_only=True)
 class Basket(Aggregate):
   basket_id: BasketId
   items: dict[ProductId, Product] = field(default_factory=dict)
@@ -228,7 +226,6 @@ ProductId = NewType('ProductId', int)
 BasketId = NewType('BasketId', int)
 
 
-@dataclass(kw_only=True)
 class Product(Entity):
   product_id: ProductId
   name: ProductName
@@ -240,7 +237,6 @@ class Basket(Aggregate):
   basket_id: BasketId
   items: dict[ProductId, Product] = field(default_factory=dict)
 
-  @dataclass(frozen=True, kw_only=True)
   class Created(AggregateEvent):
     """Basket created event"""
 
diff --git a/pyproject.toml b/pyproject.toml
index 7015931..074a5e4 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -305,6 +305,7 @@ reportUnnecessaryIsInstance = false
 # reportUnknownArgumentType = false
 reportUninitializedInstanceVariable = false
 reportUnusedParameter = false
+reportUntypedFunctionDecorator = false
 
 [tool.pytest.ini_options]
 python_files = ["*.py"]
diff --git a/src/dddkit/dataclasses/aggregates.py b/src/dddkit/dataclasses/aggregates.py
index d7754d3..3a74876 100644
--- a/src/dddkit/dataclasses/aggregates.py
+++ b/src/dddkit/dataclasses/aggregates.py
@@ -6,7 +6,6 @@
 
     BasketId = NewType('BasketId', UUID)
 
-    @dataclass(kw_only=True)
     class Basket(Aggregate):
         basket_id: BasketId
 
@@ -37,12 +36,20 @@ def delete(self) -> None:
             self.add_event(self.Deleted(basket_id=self.basket_id))
 """
 
+import inspect
 import zoneinfo
+from collections.abc import Callable, Generator
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import TypeVar
+from functools import wraps
+from types import SimpleNamespace
+from typing import ParamSpec, TypeVar, cast
 
-A = TypeVar('A', bound='Aggregate')
+from dddkit.exceptions import CheckFailedError, CheckMustReturnBoolError
+
+P = ParamSpec('P')
+R = bool | tuple[bool, str]
+T = TypeVar('T', bound=Callable[..., R])
 
 
 @dataclass(frozen=True, kw_only=True)
@@ -51,9 +58,28 @@ class AggregateEvent:
 
     occurred_on: datetime = field(default_factory=lambda: datetime.now(zoneinfo.ZoneInfo('UTC')))
 
+    def __init_subclass__(cls) -> None:
+        dataclass(cls, frozen=True, kw_only=True)
+
+
+class CheckableObject(SimpleNamespace):
+    def __post_init__(self) -> None:
+        self.__check()
+
+    def __check(self) -> None:
+        for _check in self.__obtain_checks():
+            _check()
+
+    def __obtain_checks(self) -> Generator[Callable[..., bool | tuple[bool, str]], None, None]:
+        for _method_name, method in inspect.getmembers(
+            self, predicate=lambda v: inspect.ismethod(v) and not v.__name__.startswith('_')
+        ):
+            if getattr(method, '_check', False):
+                yield cast(Callable[..., bool | tuple[bool, str]], method)
+
 
 @dataclass(kw_only=True)
-class Aggregate:
+class Aggregate(CheckableObject):
     """Aggregate.
 
     Key characteristics:
@@ -69,6 +95,9 @@ class Aggregate:
 
     _events: list[AggregateEvent] = field(default_factory=list, init=False, repr=False, compare=False)
 
+    def __init_subclass__(cls) -> None:
+        dataclass(cls, kw_only=True)
+
     def get_events(self) -> list[AggregateEvent]:
         return self._events
 
@@ -79,8 +108,7 @@ def add_event(self, event: AggregateEvent) -> None:
         self._events.append(event)
 
 
-@dataclass(kw_only=True)
-class Entity:
+class Entity(CheckableObject):
     """Entity.
 
     Key characteristics:
@@ -92,9 +120,11 @@ class Entity:
     * May contain logic
     """
 
+    def __init_subclass__(cls) -> None:
+        dataclass(cls, kw_only=True)
 
-@dataclass(frozen=True, kw_only=True)
-class ValueObject:
+
+class ValueObject(CheckableObject):
     """Value object.
 
     Key characteristics:
@@ -104,3 +134,35 @@ class ValueObject:
     * Can validate itself.
     * Can represent itself in different formats.
     """
+
+    def __init_subclass__(cls) -> None:
+        dataclass(cls, frozen=True, kw_only=True)
+
+
+def check(func: T | None = None, *, exception_type: type[Exception] | None = None) -> Callable[[T], T] | T:
+    def decorator(f: T) -> T:
+        @wraps(f)
+        def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
+            result = f(*args, **kwargs)
+            message = ''
+
+            if isinstance(result, tuple):
+                result, message = result
+
+            if not isinstance(result, bool):
+                raise CheckMustReturnBoolError
+
+            if not result:
+                if exception_type:
+                    raise exception_type(message)
+                raise CheckFailedError(message or f'Check failed: {f.__name__}')
+
+            return result
+
+        wrapper._check = True  # pyright: ignore[reportAttributeAccessIssue]
+
+        return wrapper  # pyright: ignore[reportReturnType]
+
+    if func is None:
+        return decorator
+    return decorator(func)
diff --git a/src/dddkit/exceptions.py b/src/dddkit/exceptions.py
index 1a4daba..f384edc 100644
--- a/src/dddkit/exceptions.py
+++ b/src/dddkit/exceptions.py
@@ -1,3 +1,6 @@
+CHECK_MUST_RETURN_BOOL_MSG = 'Invariants must return a boolean value'
+
+
 class DDDKitError(Exception):
     """Base class for all DDDKit exceptions."""
 
@@ -14,3 +17,12 @@ def __init__(self, package: str, install_package: str | None = None, extra: str
             f"`pip install 'dddkit[{extra or install_package or package}]'` to install dddkit with the required extra "
             f"or 'pip install {install_package or package}' to install the package separately"
         )
+
+
+class CheckMustReturnBoolError(DDDKitError):
+    def __init__(self) -> None:
+        super().__init__(CHECK_MUST_RETURN_BOOL_MSG)
+
+
+class CheckFailedError(DDDKitError):
+    """Error: check failed."""
diff --git a/src/dddkit/stories/hooks.py b/src/dddkit/stories/hooks.py
index b7fdb5f..ada5eed 100644
--- a/src/dddkit/stories/hooks.py
+++ b/src/dddkit/stories/hooks.py
@@ -1,3 +1,8 @@
+"""Hooks are needed to implement Domain Probe.
+
+https://martinfowler.com/articles/domain-oriented-observability.html
+"""
+
 from __future__ import annotations
 
 import time
diff --git a/tests/dataclasses/conftest.py b/tests/dataclasses/conftest.py
index 55b2681..ee9618a 100644
--- a/tests/dataclasses/conftest.py
+++ b/tests/dataclasses/conftest.py
@@ -18,19 +18,15 @@ class BasketChanged(DomainEvent):
 class Basket(Aggregate):
     basket_id: BasketId
 
-    @dataclass(frozen=True, kw_only=True)
     class Created(AggregateEvent):
         """Event for basket creation."""
 
-    @dataclass(frozen=True, kw_only=True)
     class Changed(AggregateEvent):
-        """Event for basket change."""
+        """Event for basket changed."""
 
-    @dataclass(frozen=True, kw_only=True)
     class ChangedId(Changed):
         basket_id: BasketId
 
-    @dataclass(frozen=True, kw_only=True)
     class Deleted(AggregateEvent):
         """Event for basket deletion."""
 
diff --git a/tests/dataclasses/test_dataclasses_aggregates.py b/tests/dataclasses/test_dataclasses_aggregates.py
index 54c58ab..e8fe9cc 100644
--- a/tests/dataclasses/test_dataclasses_aggregates.py
+++ b/tests/dataclasses/test_dataclasses_aggregates.py
@@ -1,6 +1,12 @@
 from typing import cast
 from uuid import uuid4
 
+import pytest
+
+from dddkit.dataclasses import ValueObject
+from dddkit.dataclasses.aggregates import Entity, check
+from dddkit.exceptions import CHECK_MUST_RETURN_BOOL_MSG, CheckFailedError, CheckMustReturnBoolError
+
 from .conftest import Basket, BasketId
 
 
@@ -26,3 +32,80 @@ def test_add_event(self, basket: Basket) -> None:
         assert (events := basket.get_events())
         assert isinstance(events[0], Basket.ChangedId)
         assert isinstance(events[0], Basket.Changed)
+
+
+class TestValueObject:
+    class Point(ValueObject):
+        lon: float
+        lat: float
+
+        @check()
+        def lat_must_be_in_range(self) -> bool:
+            return -90 <= self.lat <= 90
+
+        @check
+        def lon_must_be_in_range(self) -> tuple[bool, str]:
+            return -180 <= self.lon <= 180, 'Longitude must be in the range -180 to 180'
+
+        @check
+        def lat_must_be_precision_to_4(self):
+            return self.check_precision(self.lat, 4)
+
+        @check
+        def lon_must_be_precision_to_4(self):
+            return self.check_precision(self.lon, 4)
+
+        def check_precision(self, val: float, max_val: int) -> bool:
+            s = f'{val:.5f}'
+            decimal_part = s.rstrip('0').split('.')[-1]
+            return len(decimal_part) <= max_val
+
+    class PointError(ValueObject):
+        lon: float
+        lat: float
+
+        @check  # pyright: ignore[reportArgumentType]
+        def must_be_error(self) -> list:  # pyright: ignore[reportUnknownParameterType,reportMissingTypeArgument]
+            return []  # pyright: ignore[reportUnknownVariableType]
+
+    class PointCustomError(ValueObject):
+        lon: float
+        lat: float
+
+        @check(exception_type=ValueError)
+        def must_be_error(self) -> tuple[bool, str]:
+            return False, 'Custom error message'
+
+    @pytest.fixture
+    def point(self) -> Point:
+        return self.Point(lon=2.2945, lat=48.8584)
+
+    def test_value_object(self) -> None:
+        assert self.Point(lon=2.2945, lat=48.8584)
+
+    def test_value_object_not_valid(self) -> None:
+        with pytest.raises(CheckFailedError, match='Longitude must be in the range -180 to 180'):
+            self.Point(lon=200.0, lat=48.8584)
+
+    def test_value_object_with_invalid_check_return(self) -> None:
+        with pytest.raises(CheckMustReturnBoolError, match=CHECK_MUST_RETURN_BOOL_MSG):
+            self.PointError(lon=2.2945, lat=48.8584)
+
+    def test_value_object_with_custom_error(self) -> None:
+        with pytest.raises(ValueError, match='Custom error message'):
+            self.PointCustomError(lon=2.2945, lat=48.8584)
+
+
+class TestEntity:
+    class Customer(Entity):
+        first_name: str
+        last_name: str
+        age: int
+
+        @check
+        def customer_must_be_adult(self) -> bool:
+            return self.age >= 18
+
+    def test_entity_check(self):
+        with pytest.raises(CheckFailedError, match='customer_must_be_adult'):
+            self.Customer(first_name='Pete', last_name='Hodgson', age=17)
diff --git a/tests/stories/test_prometheus_hook.py b/tests/stories/test_prometheus_hook.py
index 22a3616..07c7edc 100644
--- a/tests/stories/test_prometheus_hook.py
+++ b/tests/stories/test_prometheus_hook.py
@@ -27,6 +27,17 @@ class ExpectedData(TypedDict):
     labels_step: dict[str, str]
 
 
+@pytest.fixture
+def mock_context_story(
+    sample_story: SampleStory, prometheus_hook: AIOPrometheusMetricsHook, mocker: MockerFixture
+) -> StoryExecutionContext:
+    ctx = StoryExecutionContext(story=sample_story)
+    mocker.patch.object(SampleStory, '__context__cls__', autospec=True, return_value=ctx)
+
+    sample_story.register_hook('after', prometheus_hook.after)
+    return ctx
+
+
 class TestPrometheusMetricsHook:
     @pytest.fixture
     def prometheus_hook_factory(self) -> Generator[Callable[..., PrometheusMetricsHook], None, None]:
@@ -77,16 +88,6 @@ def expected_data(self, init_data: dict[str, Any]) -> dict[str, Any]:
             },
         }
 
-    @pytest.fixture
-    def mock_context_story(
-        self, sample_story: SampleStory, prometheus_hook: PrometheusMetricsHook, mocker: MockerFixture
-    ) -> StoryExecutionContext:
-        ctx = StoryExecutionContext(story=sample_story)
-        mocker.patch.object(SampleStory, '__context__cls__', autospec=True, return_value=ctx)
-
-        sample_story.register_hook('after', prometheus_hook.after)
-        return ctx
-
     def test_hook_initialization(self, prometheus_hook: PrometheusMetricsHook, expected_data: dict[str, Any]) -> None:
         metric_name = expected_data.get('_metric_name')
         step_metric_name = expected_data.get('_step_metric_name') or ''
@@ -178,16 +179,6 @@ def expected_data(self, init_data: dict[str, Any]) -> ExpectedData:
             labels_step={'story_name': 'SampleStory', 'step_name': ANY, 'status': ANY},
         )
 
-    @pytest.fixture
-    def mock_context_story(
-        self, sample_story: SampleStory, prometheus_hook: AIOPrometheusMetricsHook, mocker: MockerFixture
-    ) -> StoryExecutionContext:
-        ctx = StoryExecutionContext(story=sample_story)
-        mocker.patch.object(SampleStory, '__context__cls__', autospec=True, return_value=ctx)
-
-        sample_story.register_hook('after', prometheus_hook.after)
-        return ctx
-
     def test_hook_initialization(self, prometheus_hook: AIOPrometheusMetricsHook, expected_data: ExpectedData) -> None:
         metric_name = expected_data.get('_metric_name')
         step_metric_name = expected_data.get('_step_metric_name') or ''