The Restatement (Second) and Restatement (Third) of the Foreign Relations Law took notably different approaches to treaty interpretation, reflecting intervening changes in the legal landscape. This symposium contribution identifies five developments in international and domestic law since the Restatement (Third). It then considers their import for the forthcoming Restatement (Fourth). Most importantly, it argues that the Restatement (Fourth) should fully incorporate two articles on treaty interpretation from the Vienna Convention on the Law of Treaties into its black-letter provisions. Since the time of the Restatement (Third), these articles have become central to international practice on treaty interpretation, and the principles they set forth are broadly consistent with how the U.S. Supreme Court approaches treaty interpretation. This contribution also suggests that the Restatement (Fourth) soften the Restatement (Third)’s provisions on deference to the executive branch in treaty interpretation. Finally, this contribution notes the rising importance in foreign relations law of the interpretation of legislation related to treaty implementation.