diff --git a/stacks-queues/stack.ipynb b/stacks-queues/stack.ipynb
index 7148988..44df098 100644
--- a/stacks-queues/stack.ipynb
+++ b/stacks-queues/stack.ipynb
@@ -13,10 +13,11 @@
    "source": [
     "## Problem: Implement a stack with push, pop, peek, and is_empty methods using a linked list.\n",
     "\n",
-    "* [Clarifying Questions](#Clarifying-Questions)\n",
+    "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
     "* [Test Cases](#Test-Cases)\n",
     "* [Algorithm](#Algorithm)\n",
     "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)\n",
     "* [Pythonic-Code](#Pythonic-Code)"
    ]
   },
@@ -24,7 +25,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Clarifying Questions\n",
+    "## Constraints and Assumptions\n",
+    "\n",
+    "*Problem statements are often intentionally ambiguous.  Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
     "\n",
     "* None"
    ]
@@ -112,11 +115,19 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 1,
    "metadata": {
     "collapsed": false
    },
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Overwriting stack.py\n"
+     ]
+    }
+   ],
    "source": [
     "%%writefile stack.py\n",
     "\n",
@@ -152,7 +163,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 2,
    "metadata": {
     "collapsed": false
    },
@@ -161,36 +172,70 @@
     "%run stack.py"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test\n",
+    "\n",
+    "*It is important to identify and run through general and edge cases from the [Test Cases](#Test-Cases) section by hand.  You generally will not be asked to write a unit test like what is shown below.*"
+   ]
+  },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 3,
    "metadata": {
     "collapsed": false
    },
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Test: Empty stack\n",
+      "Test: One element\n",
+      "Test: More than one element\n",
+      "Success: test_end_to_end\n"
+     ]
+    }
+   ],
    "source": [
-    "print('Empty stack')\n",
-    "stack = Stack()\n",
-    "print(stack.peek())\n",
-    "print(stack.pop())\n",
-    "print('One element')\n",
-    "top = Node(5)\n",
-    "stack = Stack(top)\n",
-    "print(stack.pop())\n",
-    "print(stack.peek())\n",
-    "print('More than one element')\n",
-    "stack = Stack()\n",
-    "stack.push(1)\n",
-    "stack.push(2)\n",
-    "stack.push(3)\n",
-    "print(stack.pop())\n",
-    "print(stack.peek())\n",
-    "print(stack.pop())\n",
-    "print(stack.peek())\n",
-    "print(stack.is_empty())\n",
-    "print(stack.pop())\n",
-    "print(stack.peek())\n",
-    "print(stack.is_empty())"
+    "from nose.tools import assert_equal\n",
+    "\n",
+    "class Test(object):\n",
+    "    # TODO: It would be better if we had unit tests for each\n",
+    "    # method in addition to the following end-to-end test\n",
+    "    def test_end_to_end(self):\n",
+    "        print('Test: Empty stack')\n",
+    "        stack = Stack()\n",
+    "        assert_equal(stack.peek(), None)\n",
+    "        assert_equal(stack.pop(), None)\n",
+    "\n",
+    "        print('Test: One element')\n",
+    "        top = Node(5)\n",
+    "        stack = Stack(top)\n",
+    "        assert_equal(stack.pop(), 5)\n",
+    "        assert_equal(stack.peek(), None)\n",
+    "\n",
+    "        print('Test: More than one element')\n",
+    "        stack = Stack()\n",
+    "        stack.push(1)\n",
+    "        stack.push(2)\n",
+    "        stack.push(3)\n",
+    "        assert_equal(stack.pop(), 3)\n",
+    "        assert_equal(stack.peek(), 2)\n",
+    "        assert_equal(stack.pop(), 2)\n",
+    "        assert_equal(stack.peek(), 1)\n",
+    "        assert_equal(stack.is_empty(), False)\n",
+    "        assert_equal(stack.pop(), 1)\n",
+    "        assert_equal(stack.peek(), None)\n",
+    "        assert_equal(stack.is_empty(), True)\n",
+    "        \n",
+    "        print('Success: test_end_to_end')\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    test = Test()\n",
+    "    test.test_end_to_end()"
    ]
   },
   {