On 3 December 2001, the International Tribunal for the Law of the Sea (ITLOS) issued an Order in response to Ireland's request for the prescription of provisional measures in accordance with Article 290 of the United Nations Convention on the Law of the Sea (UNCLOS). In its request, Ireland alleged violation by the United Kingdom of numerous provisions of UNCLOS. The scope of provisional measures requested by Ireland included, among others, the immediate suspension by the United Kingdom of the authorization of the Sellafield Mox Plant and a guarantee of no movement of radioactive substances or materials or wastes that are in any way related to the plant into or out of the waters of the Irish Sea. This article reviews the background to the dispute between Ireland and the United Kingdom over the operation of the Sellafield Mox Plant. It focuses on the various jurisdictional challenges raised before ITLOS and critically assesses the conclusions reached by the Tribunal in its Order.