diff --git a/recursion_dynamic/longest_substring/__init__.py b/recursion_dynamic/longest_substring/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/recursion_dynamic/longest_substring/longest_common_substr_challenge.ipynb b/recursion_dynamic/longest_substring/longest_common_substr_challenge.ipynb
new file mode 100644
index 0000000..77d4218
--- /dev/null
+++ b/recursion_dynamic/longest_substring/longest_common_substr_challenge.ipynb
@@ -0,0 +1,177 @@
+{
+ "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 two strings, find the longest common substring.\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",
+    "* Can we assume the inputs are valid?\n",
+    "    * No\n",
+    "* Can we assume the strings are ASCII?\n",
+    "    * Yes\n",
+    "* Is this case sensitive?\n",
+    "    * Yes\n",
+    "* Is a substring a contiguous block of chars?\n",
+    "    * Yes\n",
+    "* Do we expect a string as a result?\n",
+    "    * Yes\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* str0 or str1 is None -> Exception\n",
+    "* str0 or str1 equals 0 -> ''\n",
+    "* General case\n",
+    "\n",
+    "str0 = 'ABCDEFGHIJ'\n",
+    "str1 = 'FOOBCDBCDE'\n",
+    "\n",
+    "result: 'BCDE'"
+   ]
+  },
+  {
+   "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 StringCompare(object):\n",
+    "\n",
+    "    def longest_common_substr(self, str0, str1):\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_longest_common_substr.py\n",
+    "from nose.tools import assert_equal, assert_raises\n",
+    "\n",
+    "\n",
+    "class TestLongestCommonSubstr(object):\n",
+    "\n",
+    "    def test_longest_common_substr(self):\n",
+    "        str_comp = StringCompare()\n",
+    "        assert_raises(TypeError, str_comp.longest_common_substr, None, None)\n",
+    "        assert_equal(str_comp.longest_common_substr('', ''), '')\n",
+    "        str0 = 'ABCDEFGHIJ'\n",
+    "        str1 = 'FOOBCDBCDE'\n",
+    "        expected = 'BCDE'\n",
+    "        assert_equal(str_comp.longest_common_substr(str0, str1), expected)\n",
+    "        print('Success: test_longest_common_substr')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestLongestCommonSubstr()\n",
+    "    test.test_longest_common_substr()\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/recursion_dynamic/longest_substring/longest_common_substr_solution.ipynb b/recursion_dynamic/longest_substring/longest_common_substr_solution.ipynb
new file mode 100644
index 0000000..d3c4147
--- /dev/null
+++ b/recursion_dynamic/longest_substring/longest_common_substr_solution.ipynb
@@ -0,0 +1,252 @@
+{
+ "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 two strings, find the longest common substring.\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",
+    "* Can we assume the inputs are valid?\n",
+    "    * No\n",
+    "* Can we assume the strings are ASCII?\n",
+    "    * Yes\n",
+    "* Is this case sensitive?\n",
+    "    * Yes\n",
+    "* Is a substring a contiguous block of chars?\n",
+    "    * Yes\n",
+    "* Do we expect a string as a result?\n",
+    "    * Yes\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "* str0 or str1 is None -> Exception\n",
+    "* str0 or str1 equals 0 -> ''\n",
+    "* General case\n",
+    "\n",
+    "str0 = 'ABCDEFGHIJ'\n",
+    "str1 = 'FOOBCDBCDE'\n",
+    "\n",
+    "result: 'BCDE'"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll use bottom up dynamic programming to build a table.  \n",
+    "\n",
+    "<pre>\n",
+    "\n",
+    "The rows (i) represent str0.\n",
+    "The columns (j) represent str1.\n",
+    "\n",
+    "                       str1\n",
+    "  -------------------------------------------------\n",
+    "  |   |   | A | B | C | D | E | F | G | H | I | J |\n",
+    "  -------------------------------------------------\n",
+    "  |   | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |\n",
+    "  | F | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |\n",
+    "  | O | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |\n",
+    "s | O | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |\n",
+    "t | B | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |\n",
+    "r | C | 0 | 0 | 1 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |\n",
+    "0 | D | 0 | 0 | 1 | 2 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |\n",
+    "  | B | 0 | 0 | 1 | 2 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |\n",
+    "  | C | 0 | 0 | 1 | 2 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |\n",
+    "  | D | 0 | 0 | 1 | 2 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |\n",
+    "  | E | 0 | 0 | 1 | 2 | 3 | 4 | 4 | 4 | 4 | 4 | 4 |\n",
+    "  -------------------------------------------------\n",
+    "\n",
+    "if str1[j] != str0[i]:\n",
+    "    T[i][j] = max(\n",
+    "        T[i][j-1],\n",
+    "        T[i-1][j])\n",
+    "else:\n",
+    "    T[i][j] = T[i-1][j-1] + 1\n",
+    "</pre>\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(m * n), where m is the length of str0 and n is the length of str1\n",
+    "* Space: O(m * n), where m is the length of str0 and n is the length of str1"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "class StringCompare(object):\n",
+    "\n",
+    "    def longest_common_substr(self, str0, str1):\n",
+    "        if str0 is None or str1 is None:\n",
+    "            raise TypeError('str input cannot be None')\n",
+    "        # Add one to number of rows and cols for the dp table's\n",
+    "        # first row of 0's and first col of 0's\n",
+    "        num_rows = len(str0) + 1\n",
+    "        num_cols = len(str1) + 1\n",
+    "        T = [[None] * num_cols for _ in range(num_rows)]\n",
+    "        for i in range(num_rows):\n",
+    "            for j in range(num_cols):\n",
+    "                if i == 0 or j == 0:\n",
+    "                    T[i][j] = 0\n",
+    "                elif str0[j-1] != str1[i-1]:\n",
+    "                    T[i][j] = max(T[i][j-1],\n",
+    "                                  T[i-1][j])\n",
+    "                else:\n",
+    "                    T[i][j] = T[i-1][j-1] + 1\n",
+    "        results = ''\n",
+    "        i = num_rows - 1\n",
+    "        j = num_cols - 1\n",
+    "        # Walk backwards to determine the substring\n",
+    "        while T[i][j]:\n",
+    "            if T[i][j] == T[i][j-1]:\n",
+    "                j -= 1\n",
+    "            elif T[i][j] == T[i-1][j]:\n",
+    "                i -= 1\n",
+    "            elif T[i][j] == T[i-1][j-1] + 1:\n",
+    "                results += str1[i-1]\n",
+    "                i -= 1\n",
+    "                j -= 1\n",
+    "            else:\n",
+    "                raise Exception('Error constructing table')\n",
+    "        # Walking backwards results in a string in reverse order\n",
+    "        return results[::-1]                "
+   ]
+  },
+  {
+   "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_longest_common_substr.py\n"
+     ]
+    }
+   ],
+   "source": [
+    "%%writefile test_longest_common_substr.py\n",
+    "from nose.tools import assert_equal, assert_raises\n",
+    "\n",
+    "\n",
+    "class TestLongestCommonSubstr(object):\n",
+    "\n",
+    "    def test_longest_common_substr(self):\n",
+    "        str_comp = StringCompare()\n",
+    "        assert_raises(TypeError, str_comp.longest_common_substr, None, None)\n",
+    "        assert_equal(str_comp.longest_common_substr('', ''), '')\n",
+    "        str0 = 'ABCDEFGHIJ'\n",
+    "        str1 = 'FOOBCDBCDE'\n",
+    "        expected = 'BCDE'\n",
+    "        assert_equal(str_comp.longest_common_substr(str0, str1), expected)\n",
+    "        print('Success: test_longest_common_substr')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestLongestCommonSubstr()\n",
+    "    test.test_longest_common_substr()\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_longest_common_substr\n"
+     ]
+    }
+   ],
+   "source": [
+    "%run -i test_longest_common_substr.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
+}
diff --git a/recursion_dynamic/longest_substring/test_longest_common_substr.py b/recursion_dynamic/longest_substring/test_longest_common_substr.py
new file mode 100644
index 0000000..8af8fe7
--- /dev/null
+++ b/recursion_dynamic/longest_substring/test_longest_common_substr.py
@@ -0,0 +1,23 @@
+from nose.tools import assert_equal, assert_raises
+
+
+class TestLongestCommonSubstr(object):
+
+    def test_longest_common_substr(self):
+        str_comp = StringCompare()
+        assert_raises(TypeError, str_comp.longest_common_substr, None, None)
+        assert_equal(str_comp.longest_common_substr('', ''), '')
+        str0 = 'ABCDEFGHIJ'
+        str1 = 'FOOBCDBCDE'
+        expected = 'BCDE'
+        assert_equal(str_comp.longest_common_substr(str0, str1), expected)
+        print('Success: test_longest_common_substr')
+
+
+def main():
+    test = TestLongestCommonSubstr()
+    test.test_longest_common_substr()
+
+
+if __name__ == '__main__':
+    main()
\ No newline at end of file