The legality of preemptive strikes is one of the most controversial questions of contemporary international law. At the core of this controversy stands the temporal dimension of self-defence: when and for how long can a state defend itself against an armed attack? Can it resort to armed force before such an attack occurs? Is anticipatory action covered by the rules of self-defence or should it be treated as a different concept? This book examines whether anticipatory action in self-defence is part of customary international law and, if so, under what conditions. The pre-Charter concept of anticipatory action is demarcated and then assessed against post-Charter state practice. Several instances of self-defence – both anticipatory and remedial – are examined to elucidate the rules governing the temporal dimension of the right. The Six-Day War (1967), the Israeli bombing of an Iraqi reactor (1981), the US invasion of Iraq (2003) and other instances of state practice are given thorough attention.