Proportionality is widely accepted as a necessary condition of justified self-defense. What gives rise to this particular condition and what role it plays in the justification of self-defense seldom receive focused critical attention. In this paper I address the standard of proportionality applicable to personal self-defense and the role that proportionality plays in justifying the use of harmful force in self-defense. I argue against an equivalent harm view of proportionality in self-defense, and in favor of a standard of proportionality in self-defense that requires comparable seriousness and takes into account the wrong, as opposed simply to the harm that the victim is fending off. I distinguish the standard of proportionality in self-defense from proportionality in circumstances of necessity, and I discuss whether proportionality is an internal or an external constraint on the right of self-defense.