This chapter first provides a conceptual analysis of the term “self-defense.” It demonstrates that self-defense need not be aimed at averting a threatened harm, but is more specifically resistance against ongoing or imminent attacks. This has important normative implications. The chapter then turns from the definition to the normative structure of self-defense. It argues that what grounds the self-defense justification and its particular strength and scope is the fact that self-defense is an act-specific agent-relative prerogative. The precise meaning and the implications of this will be explained in detail. The chapter then discusses the limiting conditions of necessity, proportionality, and imminence as well as the subjective element. It demonstrates that the moral rationales and precise contours of these requirements can only be explained by paying attention to the often ignored interrelations between the limiting conditions as well as by invoking both a principle of reciprocity and a principle of precaution. In doing so, the chapter rejects rights forfeiture theory (but not rights forfeiture as such) – the view that the limiting conditions are internal to liability, objectivist or fact-relative accounts of justification, and accounts of proportionality that give undue weight to the value of life to the detriment of other considerations.