diff --git a/package.json b/package.json
index 7f3c254..02cfbfb 100644
--- a/package.json
+++ b/package.json
@@ -4,6 +4,7 @@
   "private": true,
   "dependencies": {
     "bootstrap": "^3.3.7",
+    "prop-types": "^15.6.0",
     "react": "^16.2.0",
     "react-dom": "^16.2.0",
     "react-scripts": "1.0.17"
diff --git a/src/components/TodoCount.js b/src/components/TodoCount.js
index 31b68dd..5b81d29 100644
--- a/src/components/TodoCount.js
+++ b/src/components/TodoCount.js
@@ -1,4 +1,5 @@
 import React from 'react';
+import PropTypes from 'prop-types';
 
 /**
  * Component to display number of todos.
@@ -18,4 +19,8 @@ const TodoCount = (props) => {
   );
 }
 
+TodoCount.propTypes = {
+  count: PropTypes.number
+}
+
 export default TodoCount;
diff --git a/src/components/TodoItem.js b/src/components/TodoItem.js
index 01729ac..ab4f052 100644
--- a/src/components/TodoItem.js
+++ b/src/components/TodoItem.js
@@ -1,4 +1,5 @@
 import React from 'react';
+import PropTypes from 'prop-types';
 
 /**
  * Each todo item.
@@ -19,4 +20,8 @@ const TodoItem = (props) => {
   )
 }
 
+TodoItem.propTypes = {
+  item: PropTypes.object
+}
+
 export default TodoItem;
diff --git a/src/components/TodoList.js b/src/components/TodoList.js
index cfd8c6b..71bbc86 100644
--- a/src/components/TodoList.js
+++ b/src/components/TodoList.js
@@ -1,4 +1,5 @@
 import React from 'react';
+import PropTypes from 'prop-types';
 
 import InputBox from './InputBox';
 import TodoItem from './TodoItem';
@@ -29,4 +30,8 @@ const TodoList = (props) => {
   )
 }
 
+TodoList.propTypes = {
+  items: PropTypes.array
+}
+
 export default TodoList;