Abstract: This chapter surveys the theory of the public enforcement of law—the use of governmental agents (regulators, inspectors, tax auditors, police, prosecutors) to detect and to sanction violators of legal rules. The theoretical core of the analysis addresses the following basic questions: Should the form of the sanction imposed on a liable party be a fine, an imprisonment term, or a combination of the two? Should the rule of liability be strict or fault-based? If violators are caught only with a probability, how should the level of the sanction be adjusted? How much of society's resources should be devoted to apprehending violators? A variety of extensions of the central theory are then examined, including: activity level; errors; the costs of imposing fines; general enforcement; marginal deterrence; the principal-agent relationship; settlements; self-reporting; repeat offenders; imperfect knowledge about the probability and magnitude of sanctions; corruption; incapacitation; costly observation of wealth; social norms; and the fairness of sanctions.