All prescriptions of behavior for individuals require enforcement. Usually the obligation to behave in a prescribed way is entered into voluntarily by explicit or implicit contract. For example, I promise to teach certain classes with designated frequency and to discuss matters which I, and possibly others, believe are relevant to the course titles. By negotiation, and in the event of its failure, by legal action, I and my employer seek to enforce the contract of employment against large departures from the promised behavior. Performance of some kinds of behavior is difficult or impossible to enforce-such as promises to be creative, noble, or steadfast in crisis-and as a result such contractual promises are either not made or enforced only when there is an uncontroversially flagrant violation. The influence upon contract, and upon economic organization generally, of the costs of enforcing various kinds of contracts has received virtually no study by economists, despite its immense potential explanatory power. When the prescribed behavior is fixed unilaterally rather than by individual agreement, we have the regulation or law, and enforcement of these unilateral rules is the subject of the present essay. Departures of actual from prescribed behavior are crimes or violations, although one could wish for a less formidable description than "criminal" to describe many of the trifling offenses or the offenses against unjust laws. My primary purpose is to construct a theory of rational enforcement, a theory which owes much to Gary Becker's major article on the subject (1968). In the conclusion the problem of explanation, as distinguished from prescription, will be commented upon.