The criminality of blackmail has been debated vigorously by legal theorists and economists for a considerable amount of time. Yet to date there appears to be no universally agreed upon theoretical rationale for making blackmail illegal. This entry reviews the theory of blackmail developed in the law and economics literature over the last 40 years and some of the critiques that were leveled against it. It argues that the social efficiency considerations emphasized in the economic model of blackmail offer a coherent unifying framework for thinking about this issue.