This paper is an exploration of the nature of what is perhaps the most widely recognized justification for inflicting harm on human beings: the appeal to defense (self-defense and other-defense). I develop and defend a rights-based account of the appeal to defense that takes into account whether and to what degree both the aggressor and his potential victim are morally responsible for the relevant threat. However, unlike most extant rights-based accounts, mine is not a forfeiture account. That is, I do not attempt to explain the permissibility of defense in terms of the aggressor’s loss of a right not to be harmed. Instead I appeal directly to the fact that defense in the relevant cases prevents the aggressor from infringing upon the rights of his potential victim. Accordingly, I call my account a “prevention account.”.