The chapter first develops a definition of “punishment,” which refers to the aims of retribution, deterrence, and the communication of condemnation. Once these aims are taken into account, the justification of punishment flows naturally from what has been said earlier in the self-defense part of the book on issues such as rights-forfeiture, the difference between forfeiture and justification, and proportionality. However, while virtually no one denies that private self-defense can be justified, the view that private severe and very harmful punishment is justified is widely reviled as “vigilantism.” The most sustained argument for this view has been provided by Christopher Heath Wellman. The chapter argues that Wellman fails to justify the state’s exclusive right or justification to punish. In addition, it argues that once the practice of punishment is justified, single acts of punishing someone who is not an offender but reasonably believed to be one can be justified on grounds of a public authority justification. This public authority justification, however, will normally only be available to public officials, not to private citizens. Thus, although private punishment can be justified, there will still remain an important moral difference between public and private punishment.