This chapter considers sadomasochistic assault. Although no offense in the criminal law is specifically labeled as such, this kind of conduct has been prosecuted under general (nonsexual) assault and battery provisions. Consent normally is allowed as a defense to charges of assault involving other kinds of (nonsexual) consensual pain infliction, such as occurs in surgery, sports, tattooing, and religious flagellation. Sadomasochistic assault prosecutions differ in that the consent or volenti defense is generally disfavored. The chapter offers an argument for why consensual sadomasochistic sex is more wrongful than these others kinds of consensual harm causing, based on the doctrine of double effect (the idea that it is permissible to cause harm as a side effect of bringing about a good result, even though it would not be permissible to cause such harm as a means to bringing about the same good end). But even if that argument is correct, it would not necessarily justify SM’s criminalization. Also considered here is the problem of how to distinguish between SM that is truly consensual and that which is not. Given the role playing it sometimes involves, there exists a possibility that without appropriate safeguards, SM might serve as a cover for what is essentially rape and sexual assault.