diff --git a/README.md b/README.md
index 6776ec8..a53d419 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,6 @@ TBD
 ## TODO
 
 - remove
-- union
 
 ## Limitations
 
@@ -51,7 +50,11 @@ Returns a new KeySet that represents the inverse Set of this one.
 
 ### `intersect(other)`
 
-Returns a new KeySet with the intersection (A ∩ B) of both Sets.
+Returns a new KeySet with the intersection (A ∩ B) of both Sets: a set that contains the elements included in both sets.
+
+### `union(other)`
+
+Returns a new KeySet with the union (A ∩ B) of both Sets: a set that contains the elements in any of the sets.
 
 ### `includes(element)`
 
diff --git a/key_set/base.py b/key_set/base.py
index 3e19daa..a9b0e3e 100644
--- a/key_set/base.py
+++ b/key_set/base.py
@@ -59,6 +59,16 @@ def clone(self) -> KeySet:
         """Returns a new KeySet that represents the same Set of this one."""
         pass
 
+    @abstractmethod
+    def intersect(self, other: KeySet) -> KeySet:
+        """Returns a new KeySet that represents the intersection (A ∩ B)."""
+        pass
+
+    @abstractmethod
+    def union(self, other: KeySet) -> KeySet:
+        """Returns a new KeySet that contains the elements of both (A U B)."""
+        pass
+
 
 class KeySetAll(KeySet):
     """Represents the ALL sets: 𝕌 (the entirety of possible keys)."""
@@ -99,6 +109,10 @@ def intersect(self, other: KeySet) -> KeySet:
         """Returns a new KeySet that represents the intersection (A ∩ B)."""
         return other.clone()
 
+    def union(self, _other: KeySet) -> KeySet:
+        """Returns a new KeySet that contains the elements of both (A U B)."""
+        return self.clone()
+
 
 class KeySetNone(KeySet):
     """Represents the NONE sets: ø (empty set)."""
@@ -139,6 +153,10 @@ def intersect(self, _other: KeySet) -> KeySetNone:
         """Returns a new KeySet that represents the intersection (A ∩ B)."""
         return self.clone()
 
+    def union(self, other: KeySet) -> KeySet:
+        """Returns a new KeySet that contains the elements of both (A U B)."""
+        return other.clone()
+
 
 class KeySetSome(KeySet):
     """Represents the SOME sets: a concrete set (`A ⊂ 𝕌`)."""
@@ -196,6 +214,20 @@ def intersect(self, other: KeySet) -> KeySet:
             return build_some_or_none(elems)
         return NotImplemented
 
+    def union(self, other: KeySet) -> KeySet:
+        """Returns a new KeySet that contains the elements of both (A U B)."""
+        if other.represents_all():
+            return other.clone()
+        if other.represents_none():
+            return self.clone()
+        if other.represents_some():
+            elems = self._elements.union(other.elements())
+            return build_some_or_none(elems)
+        if other.represents_all_except_some():
+            elems = other.elements().difference(self._elements)
+            return build_all_except_some_or_all(elems)
+        return NotImplemented
+
 
 class KeySetAllExceptSome(KeySet):
     """Represents the ALL_EXCEPT_SOME sets: the complementary of a concrete set.
@@ -256,6 +288,20 @@ def intersect(self, other: KeySet) -> KeySet:
             return build_all_except_some_or_all(elems)
         return NotImplemented
 
+    def union(self, other: KeySet) -> KeySet:
+        """Returns a new KeySet that contains the elements of both (A U B)."""
+        if other.represents_all():
+            return other.clone()
+        if other.represents_none():
+            return self.clone()
+        if other.represents_some():
+            elems = self._elements.difference(other.elements())
+            return build_all_except_some_or_all(elems)
+        if other.represents_all_except_some():
+            elems = other.elements().intersection(self._elements)
+            return build_all_except_some_or_all(elems)
+        return NotImplemented
+
 
 TS = Union[KeySetSome, KeySetNone]
 TAES = Union[KeySetAllExceptSome, KeySetAll]
diff --git a/tests/test_all.py b/tests/test_all.py
index 74ef907..48fddf5 100644
--- a/tests/test_all.py
+++ b/tests/test_all.py
@@ -62,3 +62,27 @@ def test_intersect_all_except_some(self) -> None:
     def test_includes(self) -> None:
         ks = KeySetAll()
         assert ks.includes('a')
+
+    def test_union_all(self) -> None:
+        ks = KeySetAll()
+        other = KeySetAll()
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_none(self) -> None:
+        ks = KeySetAll()
+        other = KeySetNone()
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_some(self) -> None:
+        ks = KeySetAll()
+        other = KeySetSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_all_except_some(self) -> None:
+        ks = KeySetAll()
+        other = KeySetAllExceptSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_all()
diff --git a/tests/test_all_except_some.py b/tests/test_all_except_some.py
index efa8c39..d199805 100644
--- a/tests/test_all_except_some.py
+++ b/tests/test_all_except_some.py
@@ -122,3 +122,85 @@ def test_includes_included(self) -> None:
     def test_includes_missing(self) -> None:
         ks = KeySetAllExceptSome({'a', 'b'})
         assert ks.includes('c')
+
+    def test_union_all(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetAll()
+        actual = ks.union(other)
+        assert actual.represents_all()
+        assert actual == other
+        assert actual is not other
+
+    def test_union_none(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetNone()
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_some_same_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_some_subset_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetSome({'a'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'b'}
+
+    def test_union_some_superset_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetSome({'a', 'b', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_some_with_some_common_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetSome({'a', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'b'}
+
+    def test_union_some_without_common_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetSome({'c', 'd'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_all_except_some_same_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_all_except_some_subset_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a'}
+
+    def test_union_all_except_some_superset_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a', 'b', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_all_except_some_with_some_common_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a'}
+
+    def test_union_all_except_some_without_common_keys(self) -> None:
+        ks = KeySetAllExceptSome({'a', 'b'})
+        other = KeySetAllExceptSome({'c', 'd'})
+        actual = ks.union(other)
+        assert actual.represents_all()
diff --git a/tests/test_none.py b/tests/test_none.py
index 070ee27..03e7e79 100644
--- a/tests/test_none.py
+++ b/tests/test_none.py
@@ -64,3 +64,33 @@ def test_intersect_all_except_some(self) -> None:
     def test_includes(self) -> None:
         ks = KeySetNone()
         assert not ks.includes('a')
+
+    def test_union_all(self) -> None:
+        ks = KeySetNone()
+        other = KeySetAll()
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_none(self) -> None:
+        ks = KeySetNone()
+        other = KeySetNone()
+        actual = ks.union(other)
+        assert actual.represents_none()
+
+    def test_union_some(self) -> None:
+        ks = KeySetNone()
+        other = KeySetSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b'}
+        assert actual == other
+        assert actual is not other
+
+    def test_union_all_except_some(self) -> None:
+        ks = KeySetNone()
+        other = KeySetAllExceptSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'a', 'b'}
+        assert actual == other
+        assert actual is not other
diff --git a/tests/test_some.py b/tests/test_some.py
index bb99b8e..80ca9a7 100644
--- a/tests/test_some.py
+++ b/tests/test_some.py
@@ -122,3 +122,84 @@ def test_includes_included(self) -> None:
     def test_includes_missing(self) -> None:
         ks = KeySetSome({'a', 'b'})
         assert not ks.includes('c')
+
+    def test_union_all(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetAll()
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_none(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetNone()
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_some_same_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_some_subset_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetSome({'a'})
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b'}
+
+    def test_union_some_superset_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetSome({'a', 'b', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b', 'c'}
+
+    def test_union_some_with_some_common_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetSome({'a', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b', 'c'}
+
+    def test_union_some_without_common_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetSome({'c', 'd'})
+        actual = ks.union(other)
+        assert actual.represents_some()
+        assert actual.elements() == {'a', 'b', 'c', 'd'}
+
+    def test_union_all_except_some_same_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a', 'b'})
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_all_except_some_subset_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a'})
+        actual = ks.union(other)
+        assert actual.represents_all()
+
+    def test_union_all_except_some_superset_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a', 'b', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'c'}
+
+    def test_union_all_except_some_with_some_common_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetAllExceptSome({'a', 'c'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'c'}
+
+    def test_union_all_except_some_without_common_keys(self) -> None:
+        ks = KeySetSome({'a', 'b'})
+        other = KeySetAllExceptSome({'c', 'd'})
+        actual = ks.union(other)
+        assert actual.represents_all_except_some()
+        assert actual.elements() == {'c', 'd'}