diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index d8d9c4ba24..c5763f991f 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -2,12 +2,13 @@ module.exports = File;
 
 var SHEBANG_REGEX = /^\#\!.*/;
 
-var transform = require("./transformation/transform");
-var generate  = require("./generation/generator");
-var Scope     = require("./traverse/scope");
-var util      = require("./util");
-var t         = require("./types");
-var _         = require("lodash");
+var Transformer = require("./transformation/transformer");
+var transform   = require("./transformation/transform");
+var generate    = require("./generation/generator");
+var Scope       = require("./traverse/scope");
+var util        = require("./util");
+var t           = require("./types");
+var _           = require("lodash");
 
 function File(opts) {
   this.opts = File.normaliseOptions(opts);
@@ -28,10 +29,21 @@ File.declarations = [
 ];
 
 File.normaliseOptions = function (opts) {
-  opts = _.cloneDeep(opts || {});
+  opts = opts || {};
+
+  _.each(opts.transformers, function (transformer, i) {
+    if (!(transformer instanceof Transformer)) {
+      throw new TypeError("opts.transformers[" + i + "] isn't an instance of Transformer");
+    }
+  });
+
+  opts = _.cloneDeep(opts, function (value) {
+    return value instanceof Transformer ? value : undefined;
+  });
 
   _.defaults(opts, {
     experimental: false,
+    transformers: [],
     playground:   false,
     whitespace:   true,
     blacklist:    [],
@@ -180,6 +192,10 @@ File.prototype.transform = function (ast) {
 
   var self = this;
 
+  _.each(this.opts.transformers, function (transformer) {
+    transformer.transform(self);
+  });
+
   _.each(transform.transformers, function (transformer) {
     transformer.transform(self);
   });
diff --git a/lib/6to5/index.js b/lib/6to5/index.js
index 924e08acb3..d60d457322 100644
--- a/lib/6to5/index.js
+++ b/lib/6to5/index.js
@@ -3,6 +3,10 @@ var util      = require("./util");
 var fs        = require("fs");
 var _         = require("lodash");
 
+exports.Transformer = require("./transformation/transformer");
+
+exports.types = require("./types");
+
 exports.runtime = require("./runtime");
 
 exports.register = function (opts) {