Baselines are crucial to the definition of maritime claims and the delimitation of maritime boundaries. The United Nations Convention on the Law of the Sea (LOSC) provides for several distinct types of baseline. These various baselines are discussed relative to their practical application over the past three decades. While some LOSC baseline provisions have proved to be well drafted and have led to broad compliance, the loose language contained in other baselines Articles has resulted in their being interpreted liberally. Contemporary and emerging trends and challenges are also highlighted.