This chapter examines the use of force to enforce sanctions in the absence of express authorization by the UN Security Council. After reviewing the history and background to enforcement of sanctions which primarily takes place at sea, the chapter addresses the question of what amounts to a use of force in this context, paying particular attention to whether sanctions enforcement is ‘law enforcement’ or a use of force in the sense of Article 2(4) of the UN Charter by examining the jurisprudence of the International Court of Justice and under the United Nations Law of the Sea Convention concerning forcible measures used or threatened by state authorities against vessels or oil rigs and platforms. The chapter concludes by assessing the legal bases for the use of force to enforce sanctions, including those imposed by the United Nations.