diff --git a/README.md b/README.md
index 19ca9c7..4ae2272 100644
--- a/README.md
+++ b/README.md
@@ -208,6 +208,7 @@ Challenges, solutions, and unit tests are presented in the form of **IPython/Jup
 | Create a binary search tree with minimal height from a sorted array | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_min/bst_min_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_min/bst_min_solution.ipynb) |
 | Create a linked list for each level of a binary tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_level_lists/tree_level_lists_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb) |
 | Check if a binary tree is balanced | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/check_balance/check_balance_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/check_balance/check_balance_solution.ipynb) |
+| Determine if a tree is a valid binary search tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_solution.ipynb) |
 | Implement a binary search tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst/bst_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst/bst_solution.ipynb) |
 | Implement depth-first search on a graph |  [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_dfs/dfs_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_dfs/dfs_solution.ipynb) |
 | Implement breadth-first search on a graph | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_bfs/bfs_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_bfs/bfs_solution.ipynb) |
diff --git a/graphs_trees/bst_validate/__init__.py b/graphs_trees/bst_validate/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/graphs_trees/bst_validate/bst_validate_challenge.ipynb b/graphs_trees/bst_validate/bst_validate_challenge.ipynb
new file mode 100644
index 0000000..270b384
--- /dev/null
+++ b/graphs_trees/bst_validate/bst_validate_challenge.ipynb
@@ -0,0 +1,195 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "<small><i>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).</i></small>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Challenge Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Determine if a tree is a valid binary search tree.\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 the tree have duplicates?\n",
+    "    * Yes\n",
+    "* Can we assume we already have a Node class?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "<pre>\n",
+    "Valid:\n",
+    "      5\n",
+    "    /   \\\n",
+    "   5     8\n",
+    "  / \\   /\n",
+    " 4   6 7\n",
+    "\n",
+    "Invalid:\n",
+    "      5\n",
+    "    /   \\\n",
+    "   5     8\n",
+    "  / \\   /\n",
+    " 4   9 7\n",
+    "</pre>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "Refer to the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_solution.ipynb).  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": true
+   },
+   "outputs": [],
+   "source": [
+    "%run ../bst/bst.py\n",
+    "%load ../bst/bst.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def validate_bst(node):\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_bst_validate.py\n",
+    "from nose.tools import assert_equal\n",
+    "\n",
+    "\n",
+    "class TestBstValidate(object):\n",
+    "\n",
+    "    def test_bst_validate(self):\n",
+    "        node = Node(5)\n",
+    "        insert(node, 8)\n",
+    "        insert(node, 5)\n",
+    "        insert(node, 6)\n",
+    "        insert(node, 4)\n",
+    "        insert(node, 7)\n",
+    "        assert_equal(validate_bst(node), True)\n",
+    "\n",
+    "        root = Node(5)\n",
+    "        left = Node(5)\n",
+    "        right = Node(8)\n",
+    "        invalid = Node(20)\n",
+    "        root.left = left\n",
+    "        root.right = right\n",
+    "        root.left.right = invalid\n",
+    "        assert_equal(validate_bst(root), False)\n",
+    "        print('Success: test_bst_validate')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestBstValidate()\n",
+    "    test.test_bst_validate()\n",
+    "\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    main()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Solution Notebook\n",
+    "\n",
+    "Review the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_solution.ipynb) for a discussion on algorithms and code solutions."
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.10"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/bst_validate/bst_validate_solution.ipynb b/graphs_trees/bst_validate/bst_validate_solution.ipynb
new file mode 100644
index 0000000..1ff8045
--- /dev/null
+++ b/graphs_trees/bst_validate/bst_validate_solution.ipynb
@@ -0,0 +1,228 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "<small><i>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).</i></small>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Solution Notebook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Problem: Determine if a tree is a valid binary search tree.\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 the tree have duplicates?\n",
+    "    * Yes\n",
+    "* Can we assume we already have a Node class?\n",
+    "    * Yes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Test Cases\n",
+    "\n",
+    "<pre>\n",
+    "Valid:\n",
+    "      5\n",
+    "    /   \\\n",
+    "   5     8\n",
+    "  / \\   /\n",
+    " 4   6 7\n",
+    "\n",
+    "Invalid:\n",
+    "      5\n",
+    "    /   \\\n",
+    "   5     8\n",
+    "    \\   \n",
+    "    20\n",
+    "</pre>"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm\n",
+    "\n",
+    "We'll use a recursive solution that valides left <= current < right, passing down the min and max values as we do a depth-first traversal.\n",
+    "\n",
+    "* If the node is None, return False\n",
+    "* If min is set and the node's value <= min, return False\n",
+    "* if max is set and the node's value > max, return False\n",
+    "* Recursively call the validate function on node.left, updating max\n",
+    "* Recursively call the validate function on node.right, updating min\n",
+    "    \n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(h), where h is the height of the tree"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "%run ../bst/bst.py"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "def validate_bst(node):\n",
+    "    return __validate_bst__(node, None, None)\n",
+    "\n",
+    "\n",
+    "def __validate_bst__(node, mininum, maximum):\n",
+    "    if node is None:\n",
+    "        return True\n",
+    "    if mininum is not None and node.data <= mininum:\n",
+    "        return False\n",
+    "    if maximum is not None and node.data > maximum:\n",
+    "        return False\n",
+    "    if not __validate_bst__(node.left, mininum, node.data):\n",
+    "        return False\n",
+    "    if not __validate_bst__(node.right, node.data, maximum):\n",
+    "        return False\n",
+    "    return True"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Overwriting test_bst_validate.py\n"
+     ]
+    }
+   ],
+   "source": [
+    "%%writefile test_bst_validate.py\n",
+    "from nose.tools import assert_equal\n",
+    "\n",
+    "\n",
+    "class TestBstValidate(object):\n",
+    "\n",
+    "    def test_bst_validate(self):\n",
+    "        node = Node(5)\n",
+    "        insert(node, 8)\n",
+    "        insert(node, 5)\n",
+    "        insert(node, 6)\n",
+    "        insert(node, 4)\n",
+    "        insert(node, 7)\n",
+    "        assert_equal(validate_bst(node), True)\n",
+    "\n",
+    "        root = Node(5)\n",
+    "        left = Node(5)\n",
+    "        right = Node(8)\n",
+    "        invalid = Node(20)\n",
+    "        root.left = left\n",
+    "        root.right = right\n",
+    "        root.left.right = invalid\n",
+    "        assert_equal(validate_bst(root), False)\n",
+    "\n",
+    "        print('Success: test_bst_validate')\n",
+    "\n",
+    "\n",
+    "def main():\n",
+    "    test = TestBstValidate()\n",
+    "    test.test_bst_validate()\n",
+    "\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    main()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Success: test_bst_validate\n"
+     ]
+    }
+   ],
+   "source": [
+    "%run -i test_bst_validate.py"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 2",
+   "language": "python",
+   "name": "python2"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 2
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython2",
+   "version": "2.7.10"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/bst_validate/test_bst_validate.py b/graphs_trees/bst_validate/test_bst_validate.py
new file mode 100644
index 0000000..059baed
--- /dev/null
+++ b/graphs_trees/bst_validate/test_bst_validate.py
@@ -0,0 +1,33 @@
+from nose.tools import assert_equal
+
+
+class TestBstValidate(object):
+
+    def test_bst_validate(self):
+        node = Node(5)
+        insert(node, 8)
+        insert(node, 5)
+        insert(node, 6)
+        insert(node, 4)
+        insert(node, 7)
+        assert_equal(validate_bst(node), True)
+
+        root = Node(5)
+        left = Node(5)
+        right = Node(8)
+        invalid = Node(20)
+        root.left = left
+        root.right = right
+        root.left.right = invalid
+        assert_equal(validate_bst(root), False)
+
+        print('Success: test_bst_validate')
+
+
+def main():
+    test = TestBstValidate()
+    test.test_bst_validate()
+
+
+if __name__ == '__main__':
+    main()
\ No newline at end of file