When states legalised the maritime domain in the 20th century, the relationship between states and maritime space changed. Since the turn of the millennium, certain global trends have further amplified the role of the oceans in international affairs. This has led to a renewed focus on maritime space, as well as states' rights and responsibilities within this domain, delineated through the concept of a ‘boundary’ at sea. What, in essence, is a maritime boundary? Why do states end up disputing them? Perhaps more important, how do states go about settling such disputes, and how can we better understand the development of the legal and political principles that frame such endeavours? These are the questions examined in this article, which sets out to examine the concept of maritime boundaries and related disputes. Leaning on political science, international law and political geography, it reviews how the idea of a maritime boundary came about; what principles govern how they are drawn; how they at times are resolved; and possible future trends that might impact boundary-making at sea.