diff --git a/CHANGES.md b/CHANGES.md
index 471f1442..930efa33 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,5 +1,9 @@
 # Unreleased (in `master`)
 
+## Added
+
+- You can now provide a [`karma.config()` function](https://github.com/insin/nwb/blob/master/docs/Configuration.md#config-function-1) which will be given the generated Karma config to do whatever it wants with [[#408](https://github.com/insin/nwb/issues/408)]
+
 ## Dependencies
 
 - copy-webpack-plugin: v4.2.1 → [v4.2.3](https://github.com/webpack-contrib/copy-webpack-plugin/blob/master/CHANGELOG.md#423-2017-11-23)
diff --git a/docs/Configuration.md b/docs/Configuration.md
index 0a8792b4..d9621648 100644
--- a/docs/Configuration.md
+++ b/docs/Configuration.md
@@ -112,6 +112,7 @@ The configuration object can include the following properties:
   - [`karma.testContext`](#testcontext-string) - point to a Webpack context module for your tests
   - [`karma.testFiles`](#testfiles-string--arraystring) - patterns for test files
   - [`karma.extra`](#extra-object-1) - an escape hatch for extra Karma config, which will be merged into the generated config
+  - [`karma.config`](#config-function-1) - an escape hatch for manually editing the generated Karma config
 - [npm Build Configuration](#npm-build-configuration)
   - [`npm`](#npm-object)
   - [`npm.cjs`](#esmodules-boolean) - toggle creation of a CommonJS build
@@ -1030,6 +1031,25 @@ module.exports = {
 }
 ```
 
+##### `config`: `Function`
+
+Finally, if you need *complete* control, provide a `karma.config()` function which will be given the generated config.
+
+> **Note:** you *must* return a config object from this function.
+
+```js
+module.exports = {
+  karma: {
+    config(config) {
+      // Change config as you wish
+
+      // You MUST return the edited config object
+      return config
+    }
+  }
+}
+```
+
 ### npm Build Configuration
 
 By default, nwb creates ES5 and ES6 modules builds for publishing to npm.
diff --git a/src/createKarmaConfig.js b/src/createKarmaConfig.js
index 1c960dbc..3773f842 100644
--- a/src/createKarmaConfig.js
+++ b/src/createKarmaConfig.js
@@ -4,6 +4,7 @@ import merge from 'webpack-merge'
 
 import createWebpackConfig from './createWebpackConfig'
 import debug from './debug'
+import {UserError} from './errors'
 import {deepToString, typeOf} from './utils'
 
 // The following defaults are combined into a single extglob-style pattern to
@@ -165,7 +166,7 @@ export default function createKarmaConfig(args, buildConfig, pluginConfig, userC
     ]
   }
 
-  let karmaConfig = merge({
+  let karmaConfig = {
     browsers,
     coverageReporter: {
       dir: path.resolve('coverage'),
@@ -208,7 +209,22 @@ export default function createKarmaConfig(args, buildConfig, pluginConfig, userC
       noInfo: true,
       quiet: true,
     },
-  }, userKarma.extra)
+  }
+
+  // Any extra user Karma config is merged into the generated config to give
+  // them even more control.
+  if (userKarma.extra) {
+    karmaConfig = merge(karmaConfig, userKarma.extra)
+  }
+
+  // Finally, give them a chance to do whatever they want with the generated
+  // config.
+  if (typeOf(userKarma.config) === 'function') {
+    karmaConfig = userKarma.config(karmaConfig)
+    if (!karmaConfig) {
+      throw new UserError(`karma.config() in ${userConfig.path} didn't return anything - it must return the Karma config object.`)
+    }
+  }
 
   debug('karma config: %s', deepToString(karmaConfig))
   return karmaConfig
diff --git a/tests/createKarmaConfig-test.js b/tests/createKarmaConfig-test.js
index fe126904..9eb4fa14 100644
--- a/tests/createKarmaConfig-test.js
+++ b/tests/createKarmaConfig-test.js
@@ -160,4 +160,15 @@ describe('createKarmaConfig()', () => {
     expect(config.browsers).toEqual(['PhantomJS', 'Chrome'])
     expect(config.mochaReporter).toEqual({output: 'autowatch', showDiff: true})
   })
+  it('supports a karma.config() function to manually edit generated config', () => {
+    let config = createKarmaConfig({}, {}, {}, {
+      karma: {
+        config(karmaConfig) {
+          karmaConfig.browsers.push('Chrome')
+          return karmaConfig
+        }
+      }
+    })
+    expect(config.browsers).toEqual(['PhantomJS', 'Chrome'])
+  })
 })