Coastal States exercise sovereignty and sovereign rights in maritime zones, measured from their coasts. The limits to these maritime zones are bound to recede as sea levels rise and coastlines are eroded. Furthermore, ocean acidification and ocean warming are increasingly threatening coastal ecosystems, which States are obligated to protect and manage sustainably. These changes, accelerating as the planet heats, prompt an urgent need to clarify and update the international law of maritime zones. This book explains how bilateral maritime boundaries are established, and how coastal instability and vulnerable ecosystems can affect the delimitation process through bilateral negotiations or judicial settlement. Árnadóttir engages with core concepts within public international law to address emerging issues, such as diminishing territory and changing boundaries. She proposes viable ways of addressing future challenges and sets out how fundamental changes to the marine environment can justify termination or revision of settled maritime boundaries and related agreements.