Proliferation Security Initiative (PSI)

A voluntary multilateral activity, launched on 31 May 2003, to interdict shipments of weapons of mass destruction, their delivery systems, and related materials to and from states and non-state actors of proliferation concern. Endorsed by over 100 states through the Statement of Interdiction Principles (4 September 2003), it relies on existing national and international legal authorities, including flag-state consent and boarding agreements, rather than creating new law. It complements UNSC Resolution 1540 (2004) and the SUA 2005 Protocol.