diff --git a/bootstrap.php b/bootstrap.php
index 4ce26a4..1a30955 100644
--- a/bootstrap.php
+++ b/bootstrap.php
@@ -1,22 +1,26 @@
 <?php
 require('vendor/autoload.php');
 
-function ml_get_env(): MadLisp\Env
-{
-    $env = new MadLisp\Env();
-
-    $core = new MadLisp\Lib\Core();
-    $core->register($env);
-
-    return $env;
-}
-
-function ml_get_lisp(): MadLisp\Lisp
+function ml_get_lisp(): array
 {
     $tokenizer = new MadLisp\Tokenizer();
     $reader = new MadLisp\Reader();
     $eval = new MadLisp\Evaller();
     $printer = new MadLisp\Printer();
 
-    return new MadLisp\Lisp($tokenizer, $reader, $eval, $printer);
+    $lisp = new MadLisp\Lisp($tokenizer, $reader, $eval, $printer);
+
+    // environment
+
+    $env = new MadLisp\Env();
+
+    $core = new MadLisp\Lib\Core();
+    $core->register($env);
+
+    $env->set('eval', function (...$args) use ($eval, $env) {
+        $results = $eval->eval($args, $env);
+        return $results[count($results) - 1];
+    });
+
+    return [$lisp, $env];
 }
diff --git a/repl.php b/repl.php
index 191265e..5d4e211 100644
--- a/repl.php
+++ b/repl.php
@@ -1,8 +1,7 @@
 <?php
 require('bootstrap.php');
 
-$env = ml_get_env();
-$lisp = ml_get_lisp();
+list($lisp, $env) = ml_get_lisp();
 
 while (true) {
     $input = readline('> ');
diff --git a/src/Env.php b/src/Env.php
index eb3a368..8ba4d0e 100644
--- a/src/Env.php
+++ b/src/Env.php
@@ -21,8 +21,9 @@ class Env extends Hash
         throw new MadLispException("symbol $key not defined in env");
     }
 
-    public function set(string $key, $value): void
+    public function set(string $key, $value)
     {
         $this->data[$key] = $value;
+        return $value;
     }
 }
diff --git a/src/Evaller.php b/src/Evaller.php
index 1fbf2bb..2652971 100644
--- a/src/Evaller.php
+++ b/src/Evaller.php
@@ -40,21 +40,56 @@ class Evaller
 
     public function doEval($ast, Env $env)
     {
-        // Not list
+        // Not list or empty list
         if (!($ast instanceof MList)) {
             return $this->evalAst($ast, $env);
-        }
-
-        // Empty list
-        if ($ast->count() == 0) {
+        } elseif ($ast->count() == 0) {
             return $ast;
         }
 
-        $first = $ast->get(0);
-
         // Handle special keywords
-        if ($first instanceof Symbol) {
-            if ($first->getName() == 'quote') {
+        if ($ast->get(0) instanceof Symbol) {
+            if ($ast->get(0)->getName() == 'def') {
+                if ($ast->count() != 3) {
+                    throw new MadLispException("def requires exactly 2 arguments");
+                }
+
+                if (!($ast->get(1) instanceof Symbol)) {
+                    throw new MadLispException("first argument to def is not symbol");
+                }
+
+                $value = $this->doEval($ast->get(2), $env);
+                return $env->set($ast->get(1)->getName(), $value);
+            } elseif ($ast->get(0)->getName() == 'let') {
+                if ($ast->count() != 3) {
+                    throw new MadLispException("let requires exactly 2 arguments");
+                }
+
+                if (!($ast->get(1) instanceof MList)) {
+                    throw new MadLispException("first argument to let is not list");
+                }
+
+                $bindings = $ast->get(1)->getData();
+
+                if (count($bindings) % 2 == 1) {
+                    throw new MadLispException("uneven number of bindings for let");
+                }
+
+                $newEnv = new Env($env);
+
+                for ($i = 0; $i < count($bindings) - 1; $i += 2) {
+                    $key = $bindings[$i];
+
+                    if (!($key instanceof Symbol)) {
+                        throw new MadLispException("binding key for let is not symbol");
+                    }
+
+                    $val = $this->doEval($bindings[$i + 1], $newEnv);
+                    $newEnv->set($key->getName(), $val);
+                }
+
+                return $this->doEval($ast->get(2), $newEnv);
+            } elseif ($ast->get(0)->getName() == 'quote') {
                 if ($ast->count() != 2) {
                     throw new MadLispException("quote requires exactly 1 argument");
                 }
@@ -66,14 +101,12 @@ class Evaller
         // Get new evaluated list
         $ast = $this->evalAst($ast, $env);
 
-        $first = $ast->get(0);
-
-        if (!($first instanceof Closure)) {
+        // Call first argument as function
+        $func = $ast->get(0);
+        if (!($func instanceof Closure)) {
             throw new MadLispException("first item of list is not function");
         }
-
         $args = array_slice($ast->getData(), 1);
-
-        return $first(...$args);
+        return $func(...$args);
     }
 }