diff --git a/bin/6to5/index.js b/bin/6to5/index.js
index 77263e50ee..84cfa2db0b 100755
--- a/bin/6to5/index.js
+++ b/bin/6to5/index.js
@@ -17,6 +17,7 @@ commander.option("-p, --playground", "Enable playground support");
 commander.option("-m, --modules [modules]", "Module formatter type to use [common]", "common");
 commander.option("-w, --whitelist [whitelist]", "Whitelist of transformers to ONLY use", util.list);
 commander.option("-b, --blacklist [blacklist]", "Blacklist of transformers to NOT use", util.list);
+commander.option("-i, --optional [list]", "List of optional transformers to enable", util.list);
 commander.option("-o, --out-file [out]", "Compile all input files into a single file");
 commander.option("-d, --out-dir [out]", "Compile an input directory of modules into an output directory");
 commander.option("-c, --remove-comments", "Remove comments from the compiled code", false);
@@ -28,8 +29,16 @@ commander.on("--help", function(){
     console.log("  " + title + ":");
     console.log();
 
+    var hasOptional = true;
+
     _.each(_.keys(obj).sort(), function (key) {
       if (key[0] === "_") return;
+
+      if (obj[key].optional) {
+        hasOptional = true;
+        key = "[" + key + "]";
+      }
+
       console.log("    - " + key);
     });
 
@@ -96,6 +105,7 @@ exports.opts = {
   blacklist:     commander.blacklist,
   whitelist:     commander.whitelist,
   sourceMap:     commander.sourceMaps || commander.sourceMapsInline,
+  optional:      commander.optional,
   comments:      !commander.removeComments,
   runtime:       commander.runtime,
   modules:       commander.modules
diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 3f055a7456..53550d43b3 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -10,9 +10,10 @@ var t         = require("./types");
 var _         = require("lodash");
 
 function File(opts) {
-  this.opts = File.normaliseOptions(opts);
-  this.uids = {};
-  this.ast  = {};
+  this.opts         = File.normaliseOptions(opts);
+  this.transformers = this.getTransformers();
+  this.uids         = {};
+  this.ast          = {};
 }
 
 File.declarations = [
@@ -39,6 +40,7 @@ File.normaliseOptions = function (opts) {
     blacklist:    [],
     whitelist:    [],
     sourceMap:    false,
+    optional:     [],
     comments:     true,
     filename:     "unknown",
     modules:      "common",
@@ -80,10 +82,24 @@ File.normaliseOptions = function (opts) {
 
   transform._ensureTransformerNames("blacklist", opts.blacklist);
   transform._ensureTransformerNames("whitelist", opts.whitelist);
+  transform._ensureTransformerNames("optional", opts.optional);
 
   return opts;
 };
 
+File.prototype.getTransformers = function () {
+  var file = this;
+  var transformers = [];
+
+  _.each(transform.transformers, function (transformer) {
+    if (transformer.canRun(file)) {
+      transformers.push(transformer);
+    }
+  });
+
+  return transformers;
+};
+
 File.prototype.toArray = function (node, i) {
   if (t.isArrayExpression(node)) {
     return node;
@@ -182,15 +198,25 @@ File.prototype.parse = function (code) {
 };
 
 File.prototype.transform = function (ast) {
+  var self = this;
+
   this.ast = ast;
   this.scope = new Scope(ast.program);
   this.moduleFormatter = this.getModuleFormatter(this.opts.modules);
 
-  var self = this;
+  var astRun = function (key) {
+    _.each(self.transformers, function (transformer) {
+      transformer.astRun(self, key);
+    });
+  };
 
-  _.each(transform.transformers, function (transformer) {
+  astRun("enter");
+
+  _.each(this.transformers, function (transformer) {
     transformer.transform(self);
   });
+
+  astRun("exit");
 };
 
 File.prototype.generate = function () {