Congressional Research Service. June 15, 2012.
The Proliferation Security Initiative (PSI) was formed to increase international cooperation in interdicting shipments of weapons of mass destruction (WMD), their delivery systems, and related materials. The Initiative was announced by President Bush on May 31, 2003. PSI does not create a new legal framework but aims to use existing national authorities and international law to achieve its goals. Initially, 11 nations signed on to the “Statement of Interdiction Principles” that guides PSI cooperation. As of May 2012, 98 countries (plus the Holy See) have committed formally to the PSI principles, although the extent of participation may vary by country. PSI has no secretariat, but an Operational Experts Group (OEG), made up of 21 PSI participants, coordinates activities.
http://fpc.state.gov/documents/organization/193683.pdf [PDF format, 16 pages].