diff --git a/src/Bundle.js b/src/Bundle.js
index db13779..6583efc 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -7,7 +7,7 @@ import ExternalModule from './ExternalModule';
 import finalisers from './finalisers/index';
 import makeLegalIdentifier from './utils/makeLegalIdentifier';
 import ensureArray from './utils/ensureArray';
-import { defaultResolver } from './utils/resolvePath';
+import { defaultResolver, defaultExternalResolver } from './utils/resolvePath';
 import { defaultLoader } from './utils/load';
 
 function badExports ( option, keys ) {
@@ -23,7 +23,8 @@ export default class Bundle {
 		this.load = options.load || defaultLoader;
 
 		this.resolvePathOptions = {
-			external: ensureArray( options.external )
+			external: ensureArray( options.external ),
+			resolveExternal: options.resolveExternal || defaultExternalResolver
 		};
 
 		this.loadOptions = {
diff --git a/src/rollup.js b/src/rollup.js
index 09baf6f..adad83f 100644
--- a/src/rollup.js
+++ b/src/rollup.js
@@ -20,16 +20,8 @@ export function rollup ( options ) {
 					throw new Error( 'You must supply options.dest to bundle.write' );
 				}
 
-				let { code, map } = bundle.generate({
-					dest,
-					format: options.format,
-					globalName: options.globalName,
-
-					// sourcemap options
-					sourceMap: !!options.sourceMap,
-					sourceMapFile: options.sourceMapFile,
-					// sourceMapRoot: options.sourceMapRoot
-				});
+				const dest = options.dest;
+				let { code, map } = bundle.generate( options );
 
 				let promises = [ writeFile( dest, code ) ];
 
diff --git a/src/utils/resolvePath.js b/src/utils/resolvePath.js
index 08de65d..ac8a376 100644
--- a/src/utils/resolvePath.js
+++ b/src/utils/resolvePath.js
@@ -1,4 +1,5 @@
 import { dirname, isAbsolute, resolve } from 'path';
+import { readFileSync } from 'sander';
 
 export function defaultResolver ( importee, importer, options ) {
 	// absolute paths are left untouched
@@ -9,14 +10,46 @@ export function defaultResolver ( importee, importer, options ) {
 		// unless we want to keep it external, that is
 		if ( ~options.external.indexOf( importee ) ) return null;
 
-		return resolveExternal( importee, importer, options );
+		return options.resolveExternal( importee, importer, options );
 	}
 
 	return resolve( dirname( importer ), importee ).replace( /\.js$/, '' ) + '.js';
 }
 
-function resolveExternal ( id, importer, options ) {
+export function defaultExternalResolver ( id, importer, options ) {
 	// for now, only node_modules is supported, and only jsnext:main
+	let dir = dirname( importer );
 
-	throw new Error( "TODO" );
+	while ( dir !== '/' ) {
+		const pkgPath = resolve( dir, 'node_modules', id, 'package.json' );
+		let pkgJson;
+
+		try {
+			pkgJson = readFileSync( pkgPath ).toString();
+		} catch ( err ) {
+			// noop
+		}
+
+		if ( pkgJson ) {
+			let pkg;
+
+			try {
+				pkg = JSON.parse( pkgJson );
+			} catch ( err ) {
+				throw new Error( `Malformed JSON: ${pkgPath}` );
+			}
+
+			const main = pkg[ 'jsnext:main' ];
+
+			if ( !main ) {
+				throw new Error( `Package ${id} does not have a jsnext:main field, and so cannot be included in your rollup. Try adding it as an external module instead (e.g. options.external = ['${id}']). See https://github.com/rollup/rollup/wiki/jsnext:main for more info` );
+			}
+
+			return resolve( dirname( pkgPath ), main ).replace( /\.js$/, '' ) + '.js';
+		}
+
+		dir = dirname( dir );
+	}
+
+	throw new Error( `Could not find package ${id} (required by ${importer})` );
 }
diff --git a/test/function/custom-external-resolver-async/_config.js b/test/function/custom-external-resolver-async/_config.js
new file mode 100644
index 0000000..34faf37
--- /dev/null
+++ b/test/function/custom-external-resolver-async/_config.js
@@ -0,0 +1,15 @@
+var path = require( 'path' );
+var assert = require( 'assert' );
+var Promise = require( 'sander' ).Promise;
+
+module.exports = {
+	description: 'uses a custom external path resolver (asynchronous)',
+	options: {
+		resolveExternal: function ( id, importer, options ) {
+			return Promise.resolve( path.resolve( __dirname, 'js_modules', id + '.js' ) );
+		}
+	},
+	exports: function ( exports ) {
+		assert.ok( exports.success );
+	}
+};
diff --git a/test/function/custom-external-resolver-async/js_modules/external.js b/test/function/custom-external-resolver-async/js_modules/external.js
new file mode 100644
index 0000000..c1547ab
--- /dev/null
+++ b/test/function/custom-external-resolver-async/js_modules/external.js
@@ -0,0 +1 @@
+export default { isExternal: true };
diff --git a/test/function/custom-external-resolver-async/main.js b/test/function/custom-external-resolver-async/main.js
new file mode 100644
index 0000000..32214bd
--- /dev/null
+++ b/test/function/custom-external-resolver-async/main.js
@@ -0,0 +1,3 @@
+import external from 'external';
+
+export default { success: external.isExternal };
diff --git a/test/function/custom-external-resolver-sync/_config.js b/test/function/custom-external-resolver-sync/_config.js
new file mode 100644
index 0000000..2cf6de7
--- /dev/null
+++ b/test/function/custom-external-resolver-sync/_config.js
@@ -0,0 +1,14 @@
+var path = require( 'path' );
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'uses a custom external path resolver (synchronous)',
+	options: {
+		resolveExternal: function ( id, importer, options ) {
+			return path.resolve( __dirname, 'js_modules', id + '.js' );
+		}
+	},
+	exports: function ( exports ) {
+		assert.ok( exports.success );
+	}
+};
diff --git a/test/function/custom-external-resolver-sync/js_modules/external.js b/test/function/custom-external-resolver-sync/js_modules/external.js
new file mode 100644
index 0000000..c1547ab
--- /dev/null
+++ b/test/function/custom-external-resolver-sync/js_modules/external.js
@@ -0,0 +1 @@
+export default { isExternal: true };
diff --git a/test/function/custom-external-resolver-sync/main.js b/test/function/custom-external-resolver-sync/main.js
new file mode 100644
index 0000000..32214bd
--- /dev/null
+++ b/test/function/custom-external-resolver-sync/main.js
@@ -0,0 +1,3 @@
+import external from 'external';
+
+export default { success: external.isExternal };