The proposition that “promises ought to be kept is one of the most important normative ideas or value judgements in our daily lives. But what about “illegal promises”? That is to say, what about promises that are, legally or morally speaking, malum in se or inherently wrongful, such as voluntary exchanges that are inherently immoral or wrongful, like bribes, blackmail, murder, etc.? In short, what moral obligations, if any, do such promises impose? Although many of the greatest thinkers in Western civilization have offered a wide variety of theories to explain the source of promissory obligations, it turns out there is a blind spot in this centuries-old conversation, for few theorists have given the problem of illegal or immoral promises any sustained thought. Nevertheless, illegal or immoral promises should be of theoretical interest to us because such promises may help us delimit the outer boundaries of promissory obligations.