The principle of autonomy is the outcome of judicial invention. It was read into primary law by the European Court of Justice in order to enable EU law to withstand challenges from national law. Since the early 1990s, however, an external dimension of the principle has emerged clearly and, at times, forcefully. This is about protecting the distinct characteristics of the mature EU legal order from interferences originating beyond the Union. Autonomy is now associated with controversial episodes in the interactions between EU and international law (for instance, Mox Plant and Kadi and, more recently, Opinion 2/13). The lecture will explore this new life of the principle of autonomy. It will outline its development and will, then, focus on its more recent manifestation in the developing area of investment law.