Southeast Asia, consisting of the ten Member States of the Association of Southeast Asian Nations (ASEAN), has always been a uniquely maritime region. Indeed, its maritime nature has been characterised as the ‘first and primary unifying factor of Southeast Asia’. Nine out of the ten ASEAN States are coastal States, with the Philippines and Indonesia being the world’s largest archipelagic States. From the 1960s, newly independent Southeast Asian States, free of their colonial heritage, wishing to have control over their ocean resources and bolstered by ongoing negotiations of the Third UN Conference on the Law of the Sea, began to make maritime claims in the waters adjacent to their coasts. However, the congested coastal geography characterised by the presence of ‘gulfs that penetrate deeply into the mainland, a multitude of large and small islands and wide and narrow [continental] margins’, meant that nearly all Southeast Asian waters were enclosed in overlapping claims of territorial seas, exclusive economic zones (EEZs) and/or continental shelves. These overlaps have generated ‘disputes, and even conflicts, that would not have arisen in a diffuse region such as the Southwest and Central Pacific Ocean’. Further, such disputes have been exacerbated by territorial sovereignty disputes over small islands. Despite these challenges, Southeast Asia has been described as the ‘scene of very active and innovative ocean boundary diplomacy’, and a significant number of maritime boundary delimitation agreements have been concluded.