diff --git a/docs/core-concepts/building-blocks/aggregates.md b/docs/core-concepts/building-blocks/aggregates.md
new file mode 100644
index 00000000..a853c7da
--- /dev/null
+++ b/docs/core-concepts/building-blocks/aggregates.md
@@ -0,0 +1,2 @@
+# Aggregates
+
diff --git a/docs/core-concepts/building-blocks/entities.md b/docs/core-concepts/building-blocks/entities.md
index abbf80fb..43692ba3 100644
--- a/docs/core-concepts/building-blocks/entities.md
+++ b/docs/core-concepts/building-blocks/entities.md
@@ -1,3 +1,5 @@
+# Entities
+
 Entities in one domain can be a Value Object in another. For example, a seat is
 an Entity in the Theatre domain if the theatre allocate seat numbers to patrons.
 If visitors are not allotted specific seats, then a seat can be considered a
diff --git a/docs/guides/domain-definition/fields/association-fields.md b/docs/guides/domain-definition/fields/association-fields.md
index 99d306c0..b8afb8d8 100644
--- a/docs/guides/domain-definition/fields/association-fields.md
+++ b/docs/guides/domain-definition/fields/association-fields.md
@@ -1,5 +1,15 @@
 # Association Fields
 
+Association fields in Protean are designed to represent and manage
+relationships between different domain models. They facilitate the modeling of
+complex relationships, while encapsulating the technical aspects of persisting
+data.
+
+Note that while Aggregates and Entities can manage associations, they can
+never link to another Aggregate directly. Aggregates are transaction boundaries
+and no transaction should span across aggregates. Read more in
+[Aggregate concepts](../../../core-concepts/building-blocks/aggregates.md).
+
 ## `HasOne`
 
 Represents an one-to-one association between an aggregate and its entities.
diff --git a/docs/guides/domain-definition/identity.md b/docs/guides/domain-definition/identity.md
index 362679cb..a87385d9 100644
--- a/docs/guides/domain-definition/identity.md
+++ b/docs/guides/domain-definition/identity.md
@@ -1 +1,3 @@
-# Identity
\ No newline at end of file
+# Identity
+
+When IDENTITY_TYPE is INTEGER, Strings that have valid integers or UUIDs are allowed
\ No newline at end of file
diff --git a/src/protean/domain/__init__.py b/src/protean/domain/__init__.py
index e51904bf..c1e40317 100644
--- a/src/protean/domain/__init__.py
+++ b/src/protean/domain/__init__.py
@@ -561,6 +561,40 @@ def _replace_element_by_class(self, new_element_cls):
             aggregate_record.qualname
         ].cls = new_element_cls
 
+    def _validate_domain(self):
+        """A method to validate the domain for correctness, called just before the domain is activated."""
+        # Check if all references are resolved
+        if self._pending_class_resolutions:
+            raise ConfigurationError(
+                {
+                    "element": f"Unresolved references in domain {self.name}",
+                    "unresolved": self._pending_class_resolutions,
+                }
+            )
+
+        # Check if `HasOne` and `HasMany` fields are linked to entities and not aggregates
+        for _, aggregate in self.registry.aggregates.items():
+            for _, field_obj in declared_fields(aggregate.cls).items():
+                if isinstance(field_obj, (HasOne, HasMany)):
+                    if isinstance(field_obj.to_cls, str):
+                        raise IncorrectUsageError(
+                            {
+                                "element": (
+                                    f"Unresolved target `{field_obj.to_cls}` for field "
+                                    f"`{aggregate.__name__}:{field_obj.name}`"
+                                )
+                            }
+                        )
+                    if field_obj.to_cls.element_type != DomainObjects.ENTITY:
+                        raise IncorrectUsageError(
+                            {
+                                "element": (
+                                    f"Field `{field_obj.field_name}` in `{aggregate.cls.__name__}` "
+                                    "is not linked to an Entity class"
+                                )
+                            }
+                        )
+
     ######################
     # Element Decorators #
     ######################
diff --git a/src/protean/domain/context.py b/src/protean/domain/context.py
index 70c22e2d..40786619 100644
--- a/src/protean/domain/context.py
+++ b/src/protean/domain/context.py
@@ -101,6 +101,9 @@ def push(self):
         #   This call raises an exception if all references are not resolved
         self.domain._resolve_references()
 
+        # Run Validations
+        self.domain._validate_domain()
+
     def pop(self, exc=_sentinel):
         """Pops the domain context."""
         try:
diff --git a/src/protean/fields/association.py b/src/protean/fields/association.py
index 1caa7478..dbb6a5b5 100644
--- a/src/protean/fields/association.py
+++ b/src/protean/fields/association.py
@@ -258,7 +258,7 @@ class Association(FieldBase, FieldDescriptorMixin, FieldCacheMixin):
     def __init__(self, to_cls, via=None, **kwargs):
         super().__init__(**kwargs)
 
-        self.to_cls = to_cls  # FIXME Test that `to_cls` contains a corresponding `Reference` field
+        self.to_cls = to_cls
         self.via = via
 
         # FIXME Find an elegant way to avoid these declarations in associations
diff --git a/tests/aggregate/test_aggregate_abstraction.py b/tests/aggregate/test_aggregate_abstraction.py
index 3cf7af73..43b59dd4 100644
--- a/tests/aggregate/test_aggregate_abstraction.py
+++ b/tests/aggregate/test_aggregate_abstraction.py
@@ -1,6 +1,8 @@
 import pytest
 
+from protean import BaseAggregate
 from protean.exceptions import NotSupportedError
+from protean.fields import String
 from protean.reflection import declared_fields
 
 from .elements import AbstractRole, ConcreteRole
@@ -23,9 +25,6 @@ def test_that_concrete_entities_can_be_created_from_abstract_entities_through_in
         assert concrete_role.foo == "Titan"
 
     def test_that_abstract_entities_can_be_created_with_annotations(self, test_domain):
-        from protean import BaseAggregate
-        from protean.fields import String
-
         class CustomBaseClass(BaseAggregate):
             foo = String(max_length=25)
 
diff --git a/tests/context/tests.py b/tests/context/tests.py
index bb732c11..196885a0 100644
--- a/tests/context/tests.py
+++ b/tests/context/tests.py
@@ -1,3 +1,4 @@
+import mock
 import pytest
 
 from protean.globals import current_domain, g
@@ -152,3 +153,9 @@ def test_domain_context_globals_not_shared(self, test_domain):
 
             with test_domain.domain_context(foo="baz"):
                 assert g.foo == "baz"
+
+    def test_domain_context_activation_calls_validate_domain(self, test_domain):
+        mock_validate_domain = mock.Mock()
+        test_domain._validate_domain = mock_validate_domain
+        with test_domain.domain_context():
+            mock_validate_domain.assert_called_once()
diff --git a/tests/field/test_has_many.py b/tests/field/test_has_many.py
index eb338e9c..b8d1a4b8 100644
--- a/tests/field/test_has_many.py
+++ b/tests/field/test_has_many.py
@@ -1,6 +1,7 @@
 import pytest
 
 from protean import BaseAggregate, BaseEntity
+from protean.exceptions import IncorrectUsageError
 from protean.fields import HasMany, String
 from protean.reflection import attributes, declared_fields
 
@@ -23,7 +24,7 @@ def register_elements(test_domain):
     test_domain.register(Comment)
 
 
-class TestHasManyFields:
+class TestHasManyFieldInProperties:
     def test_that_has_many_field_appears_in_fields(self):
         assert "comments" in declared_fields(Post)
 
@@ -37,6 +38,17 @@ def test_that_reference_field_does_not_appear_in_attributes(self):
         assert "post" not in attributes(Comment)
 
 
+class TestHasManyField:
+    def test_that_has_many_field_cannot_be_linked_to_aggregates(self, test_domain):
+        class InvalidAggregate(BaseAggregate):
+            post = HasMany(Post)
+
+        test_domain.register(InvalidAggregate)
+        with pytest.raises(IncorrectUsageError):
+            # The `post` field is invalid because it is linked to another Aggregate
+            test_domain._validate_domain()
+
+
 class TestHasManyPersistence:
     def test_that_has_many_field_is_persisted_along_with_aggregate(self, test_domain):
         comment = Comment(content="First Comment")
diff --git a/tests/field/test_has_one.py b/tests/field/test_has_one.py
index d26c1ec9..c4093469 100644
--- a/tests/field/test_has_one.py
+++ b/tests/field/test_has_one.py
@@ -1,6 +1,7 @@
 import pytest
 
 from protean import BaseAggregate, BaseEntity
+from protean.exceptions import IncorrectUsageError
 from protean.fields import HasOne, Reference, String
 from protean.reflection import attributes, declared_fields
 
@@ -24,7 +25,7 @@ def register(test_domain):
     test_domain.register(Author)
 
 
-class TestHasOneFields:
+class TestHasOneFieldsInProperties:
     def test_that_has_one_field_appears_in_fields(self):
         assert "author" in declared_fields(Book)
 
@@ -38,6 +39,17 @@ def test_that_reference_field_does_not_appear_in_attributes(self):
         assert "book" not in attributes(Author)
 
 
+class TestHasOneField:
+    def test_that_has_one_field_cannot_be_linked_to_aggregates(self, test_domain):
+        class InvalidAggregate(BaseAggregate):
+            author = HasOne("Book")
+
+        test_domain.register(InvalidAggregate)
+        with pytest.raises(IncorrectUsageError):
+            # The `author` HasOne field is invalid because it is linked to an Aggregate
+            test_domain._validate_domain()
+
+
 class TestHasOnePersistence:
     def test_that_has_one_field_is_persisted_along_with_aggregate(self, test_domain):
         author = Author(name="John Doe")