The State Department website shows that currently 105 countries (including the United States) plus the Holy See participate in the initiative (see the Appendix ). For example, participating states are encouraged to: 
formally commit to and publicly endorse, if possible, the Statement of Principles; review and provide information on current national legal authorities and indicate willingness to strengthen authorities as appropriate; identify specific national assets that might contribute to PSI efforts; provide points of contact for interdiction requests; be willing to actively participate in PSI interdiction training exercises and actual operations as they arise; and be willing to consider signing relevant agreements or to otherwise establish a concrete basis for cooperation with PSI efforts. Organization

PSI has no international secretariat and no distinct program funding. An informal coordinating structure has developed through an Operational Experts Group (OEG), which discusses proliferation concerns and plans future exercises. Several approaches under the PSI framework may help improve interdiction efforts. While some Bush Administration officials have cited this as an example of a successful PSI interdiction, others have argued it was part of a separate operation, and thus should not be used as evidence of PSI's success. Legal Authorities

U.S. officials have been careful to emphasize that PSI actions, including ship boarding and seizures, would be carried out in accordance with national legal authorities and international law and frameworks. The letter said that the Convention supports the efforts of the Proliferation Security Initiative. In President Bush's submission note to the Senate, he summarizes the importance of this protocol to PSI activities: "The 2005 SUA Protocol also provides for a ship-boarding regime based on flag state consent that will provide an international legal basis for interdiction at sea of weapons of mass destruction, their delivery systems and related materials." Specifically, the bill dictated that,
(11) PROLIFERATION SECURITY INITIATIVE- Funds appropriated by this Act under the heading `Nonproliferation, Anti-terrorism, Demining and Related Programs' shall be made available for programs to increase international participation in the Proliferation Security Initiative (PSI) and endorsement of the PSI Statement of Interdiction Principles: Provided, That not later than 45 days after enactment of this Act, the Secretary of State shall submit a report to the Committees on Appropriations detailing steps to be taken to implement the requirements of this paragraph. PSI Participants (as of August 9, 2018)
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 June 2018, 105 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.
Although WMD interdiction efforts took place with international cooperation before PSI was formed, supporters argue that PSI training exercises and boarding agreements give a structure and expectation of cooperation that has improved interdiction efforts. Many observers believe that PSI's "strengthened political commitment of like-minded states" to cooperate on interdiction is a successful approach to counter-proliferation policy. The effort faced early criticism that said it would be difficult to measure the initiative's effectiveness, guarantee even participation, or sustain the effort over time in the absence of a formal multilateral framework. However, successive administrations have supported the PSI and worked to expand membership. Its goals have been a part of U.S. national security strategy. The 2018 Nuclear Posture Review says, "The United States will continue to work with allies and partners to disrupt proliferation networks and interdict transfers of nuclear materials and related technology."
This report will be updated as events warrant.