The Proliferation Security Initiative (PSI) At a Glance

Contact: Kelsey Davenport, Director for Nonproliferation Policy, (202) 463-8270 x102

Updated: March 2022

The Proliferation Security Initiative (PSI) is a voluntary, multilateral effort initiated by U.S. President George W. Bush in May 2003 to strengthen the nonproliferation architecture. Specifically, PSI seeks to enhance interdiction capabilities and increase coordination between states to disrupt trade in weapons of mass destruction (WMD), delivery systems, and related materials.

Several factors motivated the Bush administration to create PSI. In the 2002 National Strategy to Combat Weapons of Mass Destruction Proliferation, the administration recognized the important role that interdicting technologies and materials plays in disrupting proliferation. The strategy concluded that “we must enhance the capabilities of our military, intelligence, technical, and law enforcement communities to prevent the movement of WMD materials, technology, and expertise to hostile states and terrorist organizations.”

The Bush administration also viewed PSI as responding to a gap made evident by the December 2002 So San incident. The So San was a North Korean ship carrying Scud missiles bound for Yemen when it was intercepted by Spanish authorities acting on intelligence provided by the United States. The United States and Spain, however, could not seize the missile parts because there was no legal basis to do so. The So Sanwas released and continued on to Yemen, after Yemenis authorities provided assurances that the country would not transfer the missiles to any third party.


PSI aims to disrupt and deter shipments of biological, chemical, and nuclear weapons, their means of delivery, and the illicit transfer of dual-use goods that could be used to produce such weapons. It also seeks to enhance cooperation between states to promote intelligence and information sharing about suspected shipments of proliferation concern.

Then-Undersecretary of State for Arms Control and International Security John Bolton indicated in November 2003 that participants would be targeting shipments to non-state actors or states pursing WMDs in violation of international law.

The original PSI member states emphasized that the initiative is “an activity not an organization,” thus it has never had a formal implementing body or secretariat. PSI also does not receive dedicated funding from participating states. An informal 21-member body, known as the Operational Experts Group (OEG), serves a coordinating function and plans exercises and activities.

The list of states that comprise the OEG is available here.

Principles and Participation:

Ten countries—Australia, France, Germany, Italy, Japan, the Netherlands, Poland, Portugal, Spain, and the United Kingdom—worked with the United States to shape the initiative and craft its interdiction principles.

In September 2003, the 11 original PSI participants released the Statement of Interdiction Principles, the non-binding document that lays out the mission of PSI and the expectations for membership. Any state can join PSI by endorsing the principles.

The principles call on PSI participants, as well as other countries, to not engage in WMD-related trade with countries of proliferation concern and to permit their own vessels and aircraft to be searched if suspected of transporting such goods.

The principles also include commitments to: 

  • develop measures to interdict transfers of WMDs, and related materials and technologies of proliferation concern,
  • strengthen national legal authorities and frameworks for interdictions,
  • adopt procedures for sharing information with other states about suspected proliferation activities, and
  • support interdiction efforts, including by negotiating ship boarding consent agreements and stopping and searching suspected shipments in territorial waters and airspaces.

As of 2019, 107 states have endorsed the Statement of Principles.

Legal Status: 

PSI does not create new law, but rather relies on existing international law to conduct interdictions in international waters or airspace. For example, a ship can be stopped in international waters if it is not flying a national flag or properly registered.

States can also conduct interdictions when directed to do so by UN Security Council resolutions adopted under Section VII of the UN Charter. For example, after the UN Security Council passed two resolutions on North Korea in 2017, a group of PSI participants issued a press releasein 2018 expressing support for the interdiction provisions in the resolutions. The press release also drew attention to the complimentary actions participating states are encouraged to take that support enforcement of the resolutions.

PSI participants are encouraged to develop their national laws and join international treaties that criminalize WMD related trafficking, such as the 2005 protocol to the Convention for the Suppression of Unlawful Acts Against the Safety of Maritime Navigation(SUA Convention). The SUA convention’s 2005 protocol prohibits maritime shipment of WMDs and related technologies outside of legitimate trade under the nuclear Nonproliferation Treaty and International Atomic Energy Agency safeguards. The protocol allows ships to be boarded, with the consent of the flag state, if they are suspected of carrying illicit cargo, thus strengthening the legal basis for interdictions.

PSI member states also seek to expand their legal authority to interdict shipments by signing bilateral boarding agreements with select countries to secure expedited processes or pre-approval for stopping and searching their ships at sea.

Activities and Meetings:

To build capacity and best practices, participating states can participate in exercises and workshops convened by PSI members. Between 2003-2018 more than 85 workshops and exercises were hosted by PSI member states. Some groups of states have taken the initiative to hold more regular activities designed to counter regional threats, including:

  • The Mediterranean Initiative: During PSI’s high-level political meeting in 2013, France and Germany proposed creating a dedicated channel to focus on challenges in the Mediterranean region. The objectives of the initiative are to discuss risks specific to region, strengthen cooperation amongst PSI and non-PSI stakeholders, and increase capacity. Activities have included seminars to discuss counter-proliferation strategies for the region, table-top exercises, and live exercises that include interdiction best practices.
  • The Asia-Pacific Exercise Rotation: Australia, Japan, New Zealand, South Korea, Singapore and the United States committed to rotate hosting yearly exercises focused on the Asia-Pacific region.

A list of PSI activities from 2003-2018 is available here.

On May 28, 2013, representatives from seventy-two PSI member states held a High Level Political Meeting in Warsaw on the 10th anniversary of the PSI’s formation. Attending states affirmed four joint statements pledging to conduct “more regular and robust” PSI exercises; promote international treaties criminalizing WMD-related trafficking; share expertise and resources to enhance interdiction capabilities; and to expand “the influence of the PSI globally through outreach to new states and the public.”

A mid-level meeting took place in Washington, DC in January 2016 that included representatives from 71 countries. Thomas Countryman, U.S. assistant secretary of state for international security and nonproliferation, said that participants discussed topics such as trends in proliferation, tactics that networks use to ship sensitive materials and technologies, and options to control proliferation financing. Countryman also said that countries shared expertise and resources that should contribute to building countries’ ability to carry out interdictions.


It is difficult to assess how effective the initiative has been since its inception in 2003. Given that PSI utilizes shared intelligence and interdictions may be conducted based off of multiple streams of information, successes are rarely credited directly to the initiative.

U.S. officials have acknowledged that PSI played an important role in a number of successful interdictions, including seizing centrifuge components that the A.Q. Khan network was shipping to Libya in 2003. In a June 2006 speech, then-Undersecretary of State Robert Joseph claimed that between April 2005 and April 2006 the United States had cooperated with other PSI participants on “roughly two dozen” occasions to prevent transfers of concern. Ulrik Federspiel, Denmark’s ambassador to the United States, asserted at a May 2005 event that “the shipment of missiles has fallen significantly in the lifetime of PSI.”

At a 2008 conference for PSI participants, the United States provided a briefing paper outlining five interdictions where PSI played a significant role.

  • February 2005: A European government denied an export license for coolers U.S. intelligence assessed were intended for Iran’s nuclear program.
  • November 2006: A state stopped the transfer for chromium-nickel steel plates to Iran that could have been used for that country’s ballistic missile program and returned the materials to the country of origin.
  • February 2007: Shared intelligence led authorities in a state to interdict U.S. origin equipment bound for Syria that could have been used for ballistic missile development.
  • April 2007: An Asian country stopped a shipment of sodium perchlorate, which can be used for making solid rocket motors, that was en route to Iran.
  • June 2007: A country denied overflight to a Syrian aircraft making a trip to North Korea based on shared intelligence that the cargo was carrying ballistic missile related components.