diff --git a/keyboard/keyboard_test.go b/keyboard/keyboard_test.go
new file mode 100644
index 0000000..372d9c3
--- /dev/null
+++ b/keyboard/keyboard_test.go
@@ -0,0 +1,49 @@
+// Copyright 2019 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package keyboard
+
+import "testing"
+
+func TestString(t *testing.T) {
+	tests := []struct {
+		desc string
+		key  Key
+		want string
+	}{
+		{
+			desc: "unknown",
+			key:  Key(-1000),
+			want: "KeyUnknown",
+		},
+		{
+			desc: "defined value",
+			key:  KeyEnter,
+			want: "KeyEnter",
+		},
+		{
+			desc: "standard key",
+			key:  'a',
+			want: "a",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.desc, func(t *testing.T) {
+			if got := tc.key.String(); got != tc.want {
+				t.Errorf("String => %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
diff --git a/mouse/mouse_test.go b/mouse/mouse_test.go
new file mode 100644
index 0000000..ef20f18
--- /dev/null
+++ b/mouse/mouse_test.go
@@ -0,0 +1,44 @@
+// Copyright 2019 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package mouse
+
+import "testing"
+
+func TestString(t *testing.T) {
+	tests := []struct {
+		desc   string
+		button Button
+		want   string
+	}{
+		{
+			desc:   "unknown",
+			button: Button(-1000),
+			want:   "ButtonUnknown",
+		},
+		{
+			desc:   "defined value",
+			button: ButtonLeft,
+			want:   "ButtonLeft",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.desc, func(t *testing.T) {
+			if got := tc.button.String(); got != tc.want {
+				t.Errorf("String => %q, want %q", got, tc.want)
+			}
+		})
+	}
+}