diff --git a/lib/6to5/transform.js b/lib/6to5/transform.js
index 1233403165..9ca0d31d7f 100644
--- a/lib/6to5/transform.js
+++ b/lib/6to5/transform.js
@@ -30,12 +30,22 @@ transform.test = function (task, assert) {
   var actualAst    = actualResult.ast;
   actualCode       = recast.prettyPrint(actualAst).code;
 
-  var expectCode   = expect.code.trim();
-  var expectAst    = util.parse(expect, expectCode);
-  var expectResult = recast.prettyPrint(expectAst);
-  expectCode       = expectResult.code;
+  if (task.options.exec) {
+    try {
+      var fn = new Function("assert", actualCode);
+      fn(assert);
+    } catch (err) {
+      console.log(actualCode);
+      throw err;
+    }
+  } else {
+    var expectCode   = expect.code.trim();
+    var expectAst    = util.parse(expect, expectCode);
+    var expectResult = recast.prettyPrint(expectAst);
+    expectCode       = expectResult.code;
 
-  assert.equal(actualCode, expectCode);
+    assert.equal(actualCode, expectCode);
+  }
 
   if (task.sourceMap) {
     assert.deepEqual(task.sourceMap, actualResult.map);
diff --git a/lib/6to5/transformers/constants.js b/lib/6to5/transformers/constants.js
index 6257b8a985..ebb19b1938 100644
--- a/lib/6to5/transformers/constants.js
+++ b/lib/6to5/transformers/constants.js
@@ -18,28 +18,7 @@ exports.ForStatement = function (node) {
   _.each(node.body, function (child) {
     if (child && child.type === "VariableDeclaration" && child.kind === "const") {
       _.each(child.declarations, function (declar) {
-        var search = [declar.id];
-        var names  = [];
-
-        while (search.length) {
-          var id = search.shift();
-
-          if (id.type === "Identifier") {
-            names.push(id.name);
-          } else if (id.type === "ArrayPattern") {
-            _.each(id.elements, function (elem) {
-              search.push(elem);
-            });
-          } else if (id.type === "ObjectPattern") {
-            _.each(id.properties, function (prop) {
-              search.push(prop.value);
-            });
-          } else {
-            throw new Error("unknown node " + id.type);
-          }
-        }
-
-        _.each(names, function (name) {
+        _.each(util.getIds(declar.id), function (name) {
           check(declar, name);
           constants.push(name);
         });
diff --git a/lib/6to5/transformers/let-scoping.js b/lib/6to5/transformers/let-scoping.js
index e69de29bb2..19bd70d0cd 100644
--- a/lib/6to5/transformers/let-scoping.js
+++ b/lib/6to5/transformers/let-scoping.js
@@ -0,0 +1,68 @@
+var traverse = require("../traverse");
+var util     = require("../util");
+var b        = require("ast-types").builders;
+var _        = require("lodash");
+
+exports.VariableDeclaration = function (node, parent, file) {
+  if (node.kind !== "let") return;
+  node.kind = "var";
+
+  var ids = {};
+
+  _.each(node.declarations, function (declar) {
+    _.each(util.getIds(declar.id), function (id) {
+      ids[id] = b.identifier(file.generateUid(id));
+    });
+  });
+
+  var replaceId = function (node, parent) {
+    // not an identifier so we have no use for this node
+    if (node.type !== "Identifier") return;
+
+    // not a let reference
+    var id = ids[node.name];
+    if (!id) return;
+
+    // we're a property key
+    if (parent.type === "Property" && parent.key === node) return;
+
+    if (parent.type !== "MemberExpression") {
+      // we aren't in a member expression
+      return id;
+    } else if (parent.type === "MemberExpression" && parent.property === node && parent.computed) {
+      // we're in a member expression but we're a computed property so we're referenced
+      return id;
+    } else if (parent.type === "MemberExpression" && parent.object === node) {
+      // we're in a member expression and we're the object so we're referenced
+      return id;
+    }
+  };
+
+  var replace = function (node, parent) {
+    if (_.contains(traverse.FUNCTION_TYPES, node.type)) {
+      var letReferences = [];
+
+      traverse(node, function (node, parent) {
+        var id = replaceId(node, parent);
+        if (id) letReferences.push(id);
+        return id;
+      });
+
+      if (letReferences.length) {
+        return b.callExpression(
+          b.functionExpression(null, letReferences,
+            b.blockStatement([
+              b.returnStatement(node)
+            ]
+          )
+        ), letReferences)
+      } else {
+        return false;
+      }
+    }
+
+    return replaceId(node, parent);
+  };
+
+  traverse(parent, replace);
+};
diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 702840a495..4e042bc748 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -44,6 +44,31 @@ exports.removeProperties = function (tree) {
   return tree;
 };
 
+exports.getIds = function (node) {
+  var search = [node];
+  var ids  = [];
+
+  while (search.length) {
+    var id = search.shift();
+
+    if (id.type === "Identifier") {
+      ids.push(id.name);
+    } else if (id.type === "ArrayPattern") {
+      _.each(id.elements, function (elem) {
+        search.push(elem);
+      });
+    } else if (id.type === "ObjectPattern") {
+      _.each(id.properties, function (prop) {
+        search.push(prop.value);
+      });
+    } else {
+      throw new Error("unknown node " + id.type);
+    }
+  }
+
+  return ids;
+};
+
 exports.errorWithNode = function (node, msg) {
   var line = node.loc.start.line;
   var col  = node.loc.start.column;
diff --git a/test/fixtures/syntax/constants/block-statement/expected.js b/test/fixtures/syntax/constants/block-statement/expected.js
index 551349fa3b..9914078fb6 100644
--- a/test/fixtures/syntax/constants/block-statement/expected.js
+++ b/test/fixtures/syntax/constants/block-statement/expected.js
@@ -1,4 +1,4 @@
-for (let i in arr) {
-  let MULTIPLIER = 5;
-  console.log(arr[i] * MULTIPLIER);
+for (var _i in arr) {
+  var _MULTIPLIER = 5;
+  console.log(arr[_i] * _MULTIPLIER);
 }
diff --git a/test/fixtures/syntax/constants/destructuring/expected.js b/test/fixtures/syntax/constants/destructuring/expected.js
index 350ef2cf1a..e0990ba2d1 100644
--- a/test/fixtures/syntax/constants/destructuring/expected.js
+++ b/test/fixtures/syntax/constants/destructuring/expected.js
@@ -1,21 +1,21 @@
 var _ref = [1, 2];
-let a = _ref[0];
-let b = _ref[1];
+var _a = _ref[0];
+var _b = _ref[1];
 
 var _ref2 = [3, 4];
-let c = _ref2[0];
-let d = _ref2[1];
+var _c = _ref2[0];
+var _d = _ref2[1];
 
 var _ref3 = {
   e: 5,
   f: 6
 };
-let e = _ref3.e;
-let f = _ref3.f;
+var _e = _ref3.e;
+var _f = _ref3.f;
 
 var _ref4 = {
   a: 7,
   b: 8
 };
-let g = _ref4.a;
-let h = _ref4.b;
+var _g = _ref4.a;
+var _h = _ref4.b;
diff --git a/test/fixtures/syntax/constants/program/expected.js b/test/fixtures/syntax/constants/program/expected.js
index 7b8fde94e5..94190e5922 100644
--- a/test/fixtures/syntax/constants/program/expected.js
+++ b/test/fixtures/syntax/constants/program/expected.js
@@ -1,5 +1,5 @@
-let MULTIPLIER = 5;
+var _MULTIPLIER = 5;
 
 for (var i in arr) {
-    console.log(arr[i] * MULTIPLIER);
+  console.log(arr[i] * _MULTIPLIER);
 }
diff --git a/test/fixtures/syntax/for-of/let/expected.js b/test/fixtures/syntax/for-of/let/expected.js
index 914c1a2ec2..d7d9f90684 100644
--- a/test/fixtures/syntax/for-of/let/expected.js
+++ b/test/fixtures/syntax/for-of/let/expected.js
@@ -1,3 +1,3 @@
 for (var _iterator = arr[Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
-  let i = _step.value;
+  var _i = _step.value;
 }
diff --git a/test/fixtures/syntax/let-scoping/deep-nested/actual.js b/test/fixtures/syntax/let-scoping/deep-nested/actual.js
new file mode 100644
index 0000000000..a2884c039e
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/deep-nested/actual.js
@@ -0,0 +1,8 @@
+var a = 'var a';
+{
+  var b = 'var b';
+  {
+    var c = 'var c';
+    let d = 'let d';
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/for-in-initializers/actual.js b/test/fixtures/syntax/let-scoping/for-in-initializers/actual.js
new file mode 100644
index 0000000000..9864e590c3
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/for-in-initializers/actual.js
@@ -0,0 +1,18 @@
+var result;
+{
+  let let_result = [];
+  let let_array = ['one', 'two', 'three'];
+  for (var index in let_array) {
+    let let_index = index;
+    let let_value = let_array[let_index];
+    let_result.push(
+        function() {
+          return [let_index, let_value];
+        });
+  }
+  result = let_result;
+}
+
+assert.deepEqual(['0', 'one'], result[0]());
+assert.deepEqual(['1', 'two'], result[1]());
+assert.deepEqual(['2', 'three'], result[2]());
diff --git a/test/fixtures/syntax/let-scoping/for-initializers/actual.js b/test/fixtures/syntax/let-scoping/for-initializers/actual.js
new file mode 100644
index 0000000000..7b491b4a8d
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/for-initializers/actual.js
@@ -0,0 +1,10 @@
+var result;
+{
+  let let_x = 'let x';
+  let let_l = [];
+  for (var var_x = 1, var_y = 2, var_z = 3; var_x < 10; var_x ++) {
+     let l_x = var_x, l_y = var_y, l_z = var_z;
+     let_l.push( function() { return [ l_x, l_y, l_z ]; } );
+  }
+  result = let_l;
+}
diff --git a/test/fixtures/syntax/let-scoping/in-class/actual.js b/test/fixtures/syntax/let-scoping/in-class/actual.js
new file mode 100644
index 0000000000..4b509ca2d6
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-class/actual.js
@@ -0,0 +1,15 @@
+class LetInClass {
+  get z() {
+    let let_z = 10;
+    return let_z;
+  }
+
+  set z(v) {
+    let let_zv = v;
+  }
+
+  distance() {
+    let dist = this.y - this.x;
+    return dist;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-closure/actual.js b/test/fixtures/syntax/let-scoping/in-closure/actual.js
new file mode 100644
index 0000000000..5581d89af3
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-closure/actual.js
@@ -0,0 +1,21 @@
+function letInClosure(n) {
+  var l = [];
+  for (var i = 0; i < n; i ++) {
+    let let_i = i;
+    if (i % 3 == 0) {
+      continue;
+    }
+    l.push(function() {
+      return let_i;
+    });
+  }
+  return l;
+}
+
+var result = letInClosure(10);
+assert.deepEqual(1, result[0]());
+assert.deepEqual(2, result[1]());
+assert.deepEqual(4, result[2]());
+assert.deepEqual(5, result[3]());
+assert.deepEqual(7, result[4]());
+assert.deepEqual(8, result[5]());
diff --git a/test/fixtures/syntax/let-scoping/in-for-break-inner/actual.js b/test/fixtures/syntax/let-scoping/in-for-break-inner/actual.js
new file mode 100644
index 0000000000..dac158d86a
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for-break-inner/actual.js
@@ -0,0 +1,8 @@
+for (var x = 0; x < 10; x ++) {
+  for (var y = 0; y < 10; y ++) {
+    let z = 'let z';
+    if (x == 7) {
+      break;
+    }
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-for-break-named/actual.js b/test/fixtures/syntax/let-scoping/in-for-break-named/actual.js
new file mode 100644
index 0000000000..37c14692ba
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for-break-named/actual.js
@@ -0,0 +1,9 @@
+outer:
+for (var x = 0; x < 10; x ++) {
+  for (var y = 0; y < 10; y ++) {
+    let z = 'let z';
+    if (x == 7) {
+      break outer;
+    }
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-for-break/actual.js b/test/fixtures/syntax/let-scoping/in-for-break/actual.js
new file mode 100644
index 0000000000..69c2680c24
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for-break/actual.js
@@ -0,0 +1,6 @@
+for (var x = 0; x < 10; x ++) {
+  let y = 'let y';
+  if (x % 2 == 0) {
+    break;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-for-continue-inner/actual.js b/test/fixtures/syntax/let-scoping/in-for-continue-inner/actual.js
new file mode 100644
index 0000000000..569b022665
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for-continue-inner/actual.js
@@ -0,0 +1,8 @@
+for (var x = 0; x < 10; x ++) {
+  for (var y = 0; y < 10; y ++) {
+    let z = 'let z';
+    if (x == 7) {
+      continue;
+    }
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-for-continue-named/actual.js b/test/fixtures/syntax/let-scoping/in-for-continue-named/actual.js
new file mode 100644
index 0000000000..e1a8d96964
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for-continue-named/actual.js
@@ -0,0 +1,9 @@
+outer:
+for (var x = 0; x < 10; x ++) {
+  for (var y = 0; y < 10; y ++) {
+    let z = 'let z';
+    if (x == 7) {
+      continue outer;
+    }
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-for-continue/actual.js b/test/fixtures/syntax/let-scoping/in-for-continue/actual.js
new file mode 100644
index 0000000000..8da940c38e
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for-continue/actual.js
@@ -0,0 +1,6 @@
+for (var x = 0; x < 10; x ++) {
+  let y = 'let y';
+  if (x % 2 == 0) {
+    continue;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/in-for/in-for.js b/test/fixtures/syntax/let-scoping/in-for/in-for.js
new file mode 100644
index 0000000000..de1630e09e
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-for/in-for.js
@@ -0,0 +1,3 @@
+for (var x = 0; x < 10; x ++) {
+  let y = 'let y';
+}
diff --git a/test/fixtures/syntax/let-scoping/in-properties/actual.js b/test/fixtures/syntax/let-scoping/in-properties/actual.js
new file mode 100644
index 0000000000..bc3cfefa66
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/in-properties/actual.js
@@ -0,0 +1,15 @@
+var object = {
+  get x() {
+    while (true) {
+      let let_x = 'let x';
+      return let_x;
+    }
+  },
+
+  set x(v) {
+    do {
+      let let_v = v;
+      this.v = let_v;
+    } while (false);
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/initializer-for-1/actual.js b/test/fixtures/syntax/let-scoping/initializer-for-1/actual.js
new file mode 100644
index 0000000000..dd2909dea2
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/initializer-for-1/actual.js
@@ -0,0 +1,3 @@
+for (let x = 1; x < 10; x++) {
+  x;
+}
diff --git a/test/fixtures/syntax/let-scoping/initializer-for-2/actual.js b/test/fixtures/syntax/let-scoping/initializer-for-2/actual.js
new file mode 100644
index 0000000000..a2bd89dbac
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/initializer-for-2/actual.js
@@ -0,0 +1,4 @@
+for (let x = 1, y = 2, z = 3; x < 10; x ++) {
+  y++;
+  z++;
+}
diff --git a/test/fixtures/syntax/let-scoping/initializer-for-3/actual.js b/test/fixtures/syntax/let-scoping/initializer-for-3/actual.js
new file mode 100644
index 0000000000..4d709fa3fe
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/initializer-for-3/actual.js
@@ -0,0 +1,5 @@
+for (let x = 1, y = x + 1; x < 10 && y != 0; x ++, y *= 2) {
+  if (y > 300) {
+    continue;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/initializer-for-in/actual.js b/test/fixtures/syntax/let-scoping/initializer-for-in/actual.js
new file mode 100644
index 0000000000..b80574713d
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/initializer-for-in/actual.js
@@ -0,0 +1,5 @@
+let sum = 0;
+let a = [1,2,3];
+for (let x in a) {
+  sum = sum + a[x];
+}
diff --git a/test/fixtures/syntax/let-scoping/nested-conflict/actual.js b/test/fixtures/syntax/let-scoping/nested-conflict/actual.js
new file mode 100644
index 0000000000..808a556509
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/nested-conflict/actual.js
@@ -0,0 +1,12 @@
+(function() {
+  var x = 1;
+  function f() {
+    assert.equal(x, 1);
+    {
+      let x = 2;
+      assert.equal(x, 2);
+    }
+    assert.equal(x, 1);
+  }
+  f();
+})();
diff --git a/test/fixtures/syntax/let-scoping/nested-function-1/actual.js b/test/fixtures/syntax/let-scoping/nested-function-1/actual.js
new file mode 100644
index 0000000000..0910f17774
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/nested-function-1/actual.js
@@ -0,0 +1,5 @@
+function nestedFunction1() {
+  return function() {
+    let let_x = 'let x';
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/nested-function-2/actual.js b/test/fixtures/syntax/let-scoping/nested-function-2/actual.js
new file mode 100644
index 0000000000..aadbf8e873
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/nested-function-2/actual.js
@@ -0,0 +1,6 @@
+function nestedFunction2() {
+  let let_func = function() {
+    let let_x = 'let x';
+  }
+  return let_func;
+}
diff --git a/test/fixtures/syntax/let-scoping/nested-function-3/actual.js b/test/fixtures/syntax/let-scoping/nested-function-3/actual.js
new file mode 100644
index 0000000000..3e45d93ff5
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/nested-function-3/actual.js
@@ -0,0 +1,4 @@
+function nestedFunction3() {
+  let let_x = 'let x';
+  function function_foo() { }
+}
diff --git a/test/fixtures/syntax/let-scoping/nested/actual.js b/test/fixtures/syntax/let-scoping/nested/actual.js
new file mode 100644
index 0000000000..bf9b8f57d0
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/nested/actual.js
@@ -0,0 +1,7 @@
+var x = 'var';
+{
+  let y = 'let 1';
+}
+{
+  let y = 'let 2';
+}
diff --git a/test/fixtures/syntax/let-scoping/no-initializer-global/actual.js b/test/fixtures/syntax/let-scoping/no-initializer-global/actual.js
new file mode 100644
index 0000000000..f01f21783d
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/no-initializer-global/actual.js
@@ -0,0 +1,4 @@
+let x2;
+assert.equal(undefined, x2);
+x2 = 2;
+assert.equal(2, x2);
diff --git a/test/fixtures/syntax/let-scoping/no-initializer/actual.js b/test/fixtures/syntax/let-scoping/no-initializer/actual.js
new file mode 100644
index 0000000000..f9d28fb681
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/no-initializer/actual.js
@@ -0,0 +1,8 @@
+var x = 1;
+{
+  let x;
+  assert.equal(undefined, x);
+  x = 2;
+  assert.equal(2, x);
+}
+assert.equal(1, x);
diff --git a/test/fixtures/syntax/let-scoping/options.json b/test/fixtures/syntax/let-scoping/options.json
new file mode 100644
index 0000000000..37997efa87
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/options.json
@@ -0,0 +1,3 @@
+{
+  "exec": true
+}
diff --git a/test/fixtures/syntax/let-scoping/regress-1381/actual.js b/test/fixtures/syntax/let-scoping/regress-1381/actual.js
new file mode 100644
index 0000000000..b6f3d193a0
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/regress-1381/actual.js
@@ -0,0 +1,8 @@
+function test() {
+  let ret = true;
+  while (false)
+    for (let i = 0; i < 1; i++)
+      ret = () => i;
+  return ret
+}
+assert.ok(test());
diff --git a/test/fixtures/syntax/let-scoping/with-for-in/actual.js b/test/fixtures/syntax/let-scoping/with-for-in/actual.js
new file mode 100644
index 0000000000..e17059b7b2
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/with-for-in/actual.js
@@ -0,0 +1,6 @@
+for (var x in [1,2,3]) {
+  let let_y = x;
+  for (var for_in_z in [4,5,6]) {
+    continue;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/with-for/actual.js b/test/fixtures/syntax/let-scoping/with-for/actual.js
new file mode 100644
index 0000000000..c6c178f941
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/with-for/actual.js
@@ -0,0 +1,6 @@
+for (var x = 0; x < 10; x++) {
+  let let_y = 'let y';
+  for (var for_z = 0; for_z < 2; for_z ++) {
+    break;
+  }
+}
diff --git a/test/fixtures/syntax/let-scoping/with-switch/actual.js b/test/fixtures/syntax/let-scoping/with-switch/actual.js
new file mode 100644
index 0000000000..1df909156d
--- /dev/null
+++ b/test/fixtures/syntax/let-scoping/with-switch/actual.js
@@ -0,0 +1,12 @@
+for (var i = 0; i < 5; i ++) {
+  let let_x = 'let x';
+
+  switch (i) {
+    case 0:
+      break;
+    case 2:
+      break;
+    default:
+      break;
+  }
+}
diff --git a/test/fixtures/syntax/modules/exports-variable/expected.js b/test/fixtures/syntax/modules/exports-variable/expected.js
index fad2fb03e0..8613280809 100644
--- a/test/fixtures/syntax/modules/exports-variable/expected.js
+++ b/test/fixtures/syntax/modules/exports-variable/expected.js
@@ -9,14 +9,14 @@ exports.foo2 = foo2;
 var foo3;
 exports.foo3 = foo3;
 
-let foo4 = 2;
-exports.foo4 = foo4;
+var _foo4 = 2;
+exports.foo4 = _foo4;
 
-let foo5;
-exports.foo5 = foo5;
+var _foo5;
+exports.foo5 = _foo5;
 
-let foo6 = 3;
-exports.foo6 = foo6;
+var _foo6 = 3;
+exports.foo6 = _foo6;
 
 function foo7 () {}