Enforcement by way of unilateral economic sanctions has been described as “one of the least developed areas of international law.” The term “sanctions” is notoriously difficult to define and does not itself appear in the key international instruments. With economic sanctions regularly referred to as President Trump's “weapon of choice,” and with opposition to such measures growing, greater certainty is needed in this area of law if the legitimacy and effectiveness of sanctions are to be preserved. This essay distinguishes UN-authorized sanctions from three types of “autonomous” sanctions (collective corrective sanctions, unilateral corrective sanctions, and unilateral coercive sanctions) and argues that many uses of unilateral sanctions are either unregulated or based on questionable legality.