It is often assumed that a promise is not legally enforceable unless, as a necessary though not sufficient condition, the promise is morally ‘obligatory’ or ‘binding’. If this assumption is correct, or can be shown to be tenable, highly interesting and important things follow for both legal and contract theory. Promise-keeping emerges as a moral obligation before it is a legal one, the legal duty becoming but a sanctionable extension of the moral obligation, its continuation so to speak by other means. There is thus no fundamental break separating moral from legal promises; for even a legal promise, particularly or most visibly one not covered by existing authority, cannot be fully justified as to why it ought to be kept without support from moral grounds. In the first three sections we therefore consider the moral reasons we may advance in support of an obligation to keep promises. In the last two sections we try to relate these moral considerations more specifically to contract law.