This article examines current multilateral and bilateral efforts to interdict the maritime transport of weapons of mass destruction (WMD), their delivery systems and related 'precursors' used in their construction. The US-led Proliferation Security Initiative (PSI) has focused international attention on the proliferation of WMD, including proliferation by maritime transport. While the PSI's Statement of Interdiction Principles focuses on existing bases of jurisdiction under domestic and international law, the interdiction framework within which it operates has now been broadened. New legal bases for maritime WMD interdiction include US bilateral shipboarding agreements, the 2005 Protocol to the Suppression of Unlawful Acts Against the Safety of Maritime Navigation Convention and, potentially, UN Security Council Resolution 1540 if it affects the law of the territorial sea. Starting from a consideration of the existing framework of maritime jurisdiction, this article examines the history and likely effectiveness of these new measures, including the creation of new crimes of maritime proliferation.