This thesis examines the patterns and processes of conflict and cooperation between neighboring states as they draw their maritime boundaries in the aftermath of the Second World War. I identify, in particular, legal uncertainty as an important driver of conflicting maritime boundary claims. Legal uncertainty is high when sources of international law—conventions, custom, judicial rulings—disagree about what the law requires. When law is highly uncertain, states have opportunities and incentives to make and insist on maximalist, conflicting claims, which makes it difficult for them to delimit their boundaries. Using mixed methods and an original dataset, I find that high legal uncertainty is significantly associated with a higher probability of dispute onset, but the degree of legal uncertainty does not seem to be significantly associated with dispute duration. Surprisingly, high levels of legal uncertainty appear to be associated with an increased maritime boundary delimitation activity. To illustrate my causal mechanism and to identify factors that may account for unexpected results, I complement my large-N examination with two case studies. The case of Greece and Turkey illustrates, on the one hand, how legal uncertainty drives maximalist claims and allows states to insist on them; and on the other hand, how disputes may often outlast periods of high legal uncertainty. The Mexico-United States case attempts to tease out factors that allow states to sign delimitation agreements even when law is highly uncertain. Taken together, the statistical analyses and the case studies lay a solid foundation for further examinations into the drivers of maritime boundary activity.