In this chapter, the author examines the "temporal dimension" of self-defense. He also looks at the questions and challenges posed by the national security strategy (NSS) regulating the use of force and in those contexts he/she looks at the relationship between self-defense and the collective security system of the United Nations. With regard to anticipatory self-defense, a right to take anticipatory action has long existed under customary international law. It continues to exist under the Charter. Long-standing State practice spanning over sixty years following adoption of the Charter, together with significant evidence of the  opinio juris  in the form of the  Caroline  criteria as a set of guiding principles, support its continual vitality.  Keywords: anticipatory self-defense, Caroline criteria, customary international law, defensive armed reprisal, pre-emptive self-defense, preventive self-defense, UN collective security system, United Nations Charter law