The United Nations Convention on the Law of the Sea (LOSC) is the most important source of the international law of the sea. This chapter discusses the history and legal characteristics of the LOSC. It explains how the LOSC came into being; gives a brief overview of its provisions and considers their varying legal nature; explains which entities may and have become parties to the LOSC and considers the extent to which they are permitted to make reservations and declarations; outlines the relationship of the LOSC to other treaties and customary international law; explores the mechanisms for seeking to ensure compliance with the LOSC by its States parties; and finally discusses how the LOSC is kept under review and developed.