Among the most persistent controversies in international investment law is the nature of the “protection and security” standard found in most investment treaties. Some tribunals contend that the standard requires nothing more than physical protection of covered investments, while others maintain that it requires legal security as well. Some insist that it is entirely distinct from the fair and equitable treatment standard that is often expressed in the same sentence or paragraph, while others effectively conflate the two standards. These conflicting decisions are undermining the legitimacy of investment treaty arbitration, but this Article seeks to resolve the controversies underlying them by employing the full range of interpretive tools offered by the Vienna Convention on the Law of Treaties. It explores the text, structure, and purpose of the relevant treaties; identifies a norm of protection and security in customary international law; and traces its evolution over time. This inquiry reveals that treaty drafters have long understood protection and security as requiring a specific—and limited—form of legal security. It also reveals that fair and equitable treatment was derived from the same customary norm, but that the two standards have evolved to become conceptually distinct. The Article then employs the interpretation suggested by this analysis to critique modern treaty jurisprudence and the current U.S. approach to drafting investment treaties.