In the summer of 1884, three English sailors adrift in a lifeboat were rescued by a passing ship. The men were tremendously fortunate: they had been at sea for two weeks, and were on the brink of death from thirst and hunger. Their remarkable recovery was marred, however, by the fact that they had sustained themselves since the wreck by killing and eating one of their number. The Crown charged two of the survivors with murder. Few doubted that, considering the circumstances, all of them would have died in the boat had they not resorted to cannibalism. The question was whether their circumstances created a necessity that justified their actions. The case that followed, Regina v Dudley & Stephens, is one of the classic statements of the common law necessity defense. At common law, the necessity defense, a form of justification, permitted defendants to avoid criminal liability by appealing to a “balancing of evils.” If the defendant demonstrated that he perpetrated his crime in order to avert a greater evil, he would be acquitted. This defense was controversial at common law and poses a perennial challenge to the rule of law even as it introduces flexibility into the criminal justice system. Today, the question of whether the defense exists in modern federal criminal law remains an open question.