diff --git a/arrays_strings/two_sum/__init__.py b/arrays_strings/two_sum/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/arrays_strings/two_sum/test_two_sum.py b/arrays_strings/two_sum/test_two_sum.py
new file mode 100644
index 0000000..9f6fff3
--- /dev/null
+++ b/arrays_strings/two_sum/test_two_sum.py
@@ -0,0 +1,23 @@
+from nose.tools import assert_equal, assert_raises
+
+
+class TestTwoSum(object):
+
+    def test_two_sum(self):
+        solution = Solution()
+        assert_raises(TypeError, solution.two_sum, None, None)
+        assert_raises(ValueError, solution.two_sum, [], 0)
+        target = 7
+        nums = [1, 3, 2, -7, 5]
+        expected = [2, 4]
+        assert_equal(solution.two_sum(nums, target), expected)
+        print('Success: test_two_sum')
+
+
+def main():
+    test = TestTwoSum()
+    test.test_two_sum()
+
+
+if __name__ == '__main__':
+    main()
\ No newline at end of file
diff --git a/arrays_strings/two_sum/two_sum_challenge.ipynb b/arrays_strings/two_sum/two_sum_challenge.ipynb
new file mode 100644
index 0000000..733b9ec
--- /dev/null
+++ b/arrays_strings/two_sum/two_sum_challenge.ipynb
@@ -0,0 +1,170 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Challenge Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Given an array, find the two indices that sum to a specific value.\n",
+    "\n",
+    "* [Constraints](#Constraints)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)\n",
+    "* [Solution Notebook](#Solution-Notebook)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Constraints\n",
+    "\n",
+    "* Is there exactly one solution?\n",
+    "    * Yes\n",
+    "* Is there always a solution?\n",
+    "    * Yes\n",
+    "* Is the array an array of ints?\n",
+    "    * Yes\n",
+    "* Is the array sorted?\n",
+    "    No\n",
+    "* Are negative values possible?\n",
+    "    * Yes\n",
+    "* Can we assume the inputs are valid?\n",
+    "    * No\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* None input -> TypeError\n",
+    "* [] -> ValueError\n",
+    "* [1, 3, 2, -7, 5], 7 -> [2, 4]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "Refer to the [Solution Notebook]().  If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class Solution(object):\n",
+    "\n",
+    "    def two_sum(self, val):\n",
+    "        # TODO: Implement me\n",
+    "        pass"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**The following unit test is expected to fail until you solve the challenge.**"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "# %load test_two_sum.py\n",
+    "from nose.tools import assert_equal, assert_raises\n",
+    "\n",
+    "\n",
+    "class TestTwoSum(object):\n",
+    "\n",
+    "    def test_two_sum(self):\n",
+    "        solution = Solution()\n",
+    "        assert_raises(TypeError, solution.two_sum, None)\n",
+    "        assert_equal(solution.two_sum(0), 0)\n",
+    "        print('Success: test_two_sum')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestTwoSum()\n",
+    "    test.test_two_sum()\n",
+    "\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    main()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Solution Notebook\n",
+    "\n",
+    "Review the [Solution Notebook]() for a discussion on algorithms and code solutions."
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.5.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/arrays_strings/two_sum/two_sum_solution.ipynb b/arrays_strings/two_sum/two_sum_solution.ipynb
new file mode 100644
index 0000000..ad44d07
--- /dev/null
+++ b/arrays_strings/two_sum/two_sum_solution.ipynb
@@ -0,0 +1,271 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Solution Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Given an array, find the two indices that sum to a specific value.\n",
+    "\n",
+    "* [Constraints](#Constraints)\n",
+    "* [Test Cases](#Test-Cases)\n",
+    "* [Algorithm](#Algorithm)\n",
+    "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Constraints\n",
+    "\n",
+    "* Is there exactly one solution?\n",
+    "    * Yes\n",
+    "* Is there always a solution?\n",
+    "    * Yes\n",
+    "* Is the array an array of ints?\n",
+    "    * Yes\n",
+    "* Is the array sorted?\n",
+    "    No\n",
+    "* Are negative values possible?\n",
+    "    * Yes\n",
+    "* Can we assume the inputs are valid?\n",
+    "    * No\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* None input -> TypeError\n",
+    "* [] -> ValueError\n",
+    "* [1, 3, 2, -7, 5], 7 -> [2, 4]"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "### Brute force\n",
+    "\n",
+    "* For i in range(len(input)):\n",
+    "    * For j in range(i+1, len(input)):\n",
+    "        * if i + j == target return True\n",
+    "* return False\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n^2)\n",
+    "* Space: O(1)\n",
+    "\n",
+    "### Optimized\n",
+    "\n",
+    "<pre>\n",
+    "* Loop through each num in nums\n",
+    "    * Calculate the cache_target = target - num\n",
+    "\n",
+    "target = 7\n",
+    "index  =  0  1  2   3  4\n",
+    "nums   = [1, 3, 2, -7, 5]\n",
+    "          ^\n",
+    "cache_target = 7 - 1 = 6\n",
+    "cache\n",
+    "6 -> 0\n",
+    "\n",
+    "1 not in cache\n",
+    "\n",
+    "index  =  0  1  2   3  4\n",
+    "nums   = [1, 3, 2, -7, 5]\n",
+    "             ^\n",
+    "cache_target = 7 - 3 = 4\n",
+    "cache\n",
+    "6 -> 0\n",
+    "4 -> 1\n",
+    "\n",
+    "3 not in cache\n",
+    "\n",
+    "index  =  0  1  2   3  4\n",
+    "nums   = [1, 3, 2, -7, 5]\n",
+    "                ^\n",
+    "cache_target = 7 - 2 = 5\n",
+    "cache\n",
+    "6 -> 0\n",
+    "4 -> 1\n",
+    "5 -> 2\n",
+    "\n",
+    "2 not in cache\n",
+    "\n",
+    "index  =  0  1  2   3  4\n",
+    "nums   = [1, 3, 2, -7, 5]\n",
+    "                    ^\n",
+    "cache_target = 7 + 7 = 14\n",
+    "cache\n",
+    "6  -> 0\n",
+    "4  -> 1\n",
+    "5  -> 2\n",
+    "14 -> 3\n",
+    "\n",
+    "-7 not in cache\n",
+    "\n",
+    "index  =  0  1  2   3  4\n",
+    "nums   = [1, 3, 2, -7, 5]\n",
+    "                       ^\n",
+    "cache_target = 7 - 5 = 2\n",
+    "cache\n",
+    "6  -> 0\n",
+    "4  -> 1\n",
+    "5  -> 2\n",
+    "14 -> 3\n",
+    "\n",
+    "5 in cache, success, output matching indices: cache[num] and current iteration index\n",
+    "\n",
+    "output = [2, 4]\n",
+    "</pre>\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(n)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class Solution(object):\n",
+    "\n",
+    "    def two_sum(self, nums, target):\n",
+    "        if nums is None or target is None:\n",
+    "            raise TypeError('nums or target cannot be None')\n",
+    "        if not nums:\n",
+    "            raise ValueError('nums cannot be empty')\n",
+    "        cache = {}\n",
+    "        for index, num in enumerate(nums):\n",
+    "            cache_target = target - num\n",
+    "            if num in cache:\n",
+    "                return [cache[num], index]\n",
+    "            else:\n",
+    "                cache[cache_target] = index\n",
+    "        return None"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Overwriting test_two_sum.py\n"
+     ]
+    }
+   ],
+   "source": [
+    "%%writefile test_two_sum.py\n",
+    "from nose.tools import assert_equal, assert_raises\n",
+    "\n",
+    "\n",
+    "class TestTwoSum(object):\n",
+    "\n",
+    "    def test_two_sum(self):\n",
+    "        solution = Solution()\n",
+    "        assert_raises(TypeError, solution.two_sum, None, None)\n",
+    "        assert_raises(ValueError, solution.two_sum, [], 0)\n",
+    "        target = 7\n",
+    "        nums = [1, 3, 2, -7, 5]\n",
+    "        expected = [2, 4]\n",
+    "        assert_equal(solution.two_sum(nums, target), expected)\n",
+    "        print('Success: test_two_sum')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestTwoSum()\n",
+    "    test.test_two_sum()\n",
+    "\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    main()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Success: test_two_sum\n"
+     ]
+    }
+   ],
+   "source": [
+    "%run -i test_two_sum.py"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.5.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}