There are a number of principal arguments advanced in this work, among them that promise has played a central role in obligations theory and practice (in part, though not merely, because it has been used to describe the nature of contract), that the idea of promise as a manifestation of human will and commitment is central to an understanding of contract, that this idea explains much of the body of contractual rules and doctrines applied by the courts, and that promise narrowly defined (as a unilateral promise) is a better explanation for a number of circumstances in which voluntary obligations are intended than is the bilateral obligation of contract. However, none of these arguments can sensibly be advanced without first settling the fundamental definition of the idea of a promise and the characteristics of the practice or institution of promising. For that reason, this chapter will address some very basic matters, including: the constituent elements of a promise; how promises are formed; what the party making a promise (the promisor) must intend before a promise can exist; whether the beneficiary of a promise (the promisee) must also intend anything before the promise can be constituted; whether promises must be accepted before they bind the promisor; and whether promises may be made subject to conditions.