The waters of the Pacific, the Indian Ocean, and the South China Sea all flow through Southeast Asia, whose vast archipelagoes and extensive mainland coastlines have nurtured sea-focused cultures for millennia. Yet political and ideological legacies have led some historians of the maritime world to misrepresent people from beyond the region as the principal drivers of change, even when they were not, rather than come to grips with the enduring sea cultures at the heart of regional dynamics. This chapter highlights important recent research on the prehistory of Southeast Asian maritime networks and reviews prominent examples of polities with sea-focused cultures in the early period. It then delves into the Southeast Asian seascape that early modern European powers eventually engaged, before colonial transformations limited the political and military roles of maritime groups in regional polities (Figure 31.1).