We are in a moment where peoples and states are interested, directly or indirectly, in asserting their "national interest," unilaterally if necessary. In the White House, the national security policy is premised on "America First," while Catalans and Iraqi Kurds have taken steps to unilaterally declare their independence. All of these actions have generated tension both domestically and internationally. However, even though the potential for unilateral action has been receiving a lot of attention, the larger issue of the legality of unilateral acts is often hard to discern. This book provides a history of the doctrine of unilateral acts in international law, tracing their treatment in the international sphere from consent based acts, to obligations erga omnes, to acts of estoppel. Through chapter-by-chapter case studies, this book traces the "legalization" of the category of unilateral acts from its 19th Century foundations into a broad category of obligation. To understand why and how this occurred, this book examines the history of the legal doctrine of unilateral acts, which shows that in spite of efforts to progressively make unilateral acts "legal" they are still not precisely defined or easy to apply, challenging the very commitment these acts are meant to establish.