Global sea level rise has raised concerns over the potential inundation and erosion of low-lying coasts, including islands, around the world. Freestone was among the early commentators to identify related threats to the location of baselines along the coast and therefore on the scope of national claims to maritime jurisdiction. The chapter outlines the evolution of scientific understanding and projections of sea level rise while acknowledging the considerable associated complexities and uncertainties in the application of such projections to specific coastal situations. The implications of sea level rise for maritime claims and the evolving practice of States in response to this threat are then discussed. The chapter concludes with considerations on how this area of the law of the sea represents a ‘new frontier’, raising challenges to existing norms with a view to ensuring greater stability and certainty in the definition, delineation and delimitation baselines, maritime limits and boundaries.