This article addresses the management of maritime plunder and conflict in the waters of England and France in the fourteenth century. It argues that during this century a fundamental change occurred. Around 1300, maritime conflict was handled by recourse to the strictly civil law merchant and law maritime, or by Marcher law. However by the 1350s and 1360s the kings of England and France, moved by contemporary political events and theories of sovereignty at sea, created courts of Admiralty that challenged the previous systems’ jurisdiction. These initiatives eventually paved the way for the criminalisation of private maritime conflict.