The Proliferation Security Initiative (PSI), begun in 2003, was conceived as an activity rather than an organization. PSI's purpose is to prevent or at least inhibit the spread of weapons of mass destruction (WMD), their delivery systems, and related materials to or from states or nonstate actors whose possession of such items would be a serious threat to global or regional security. An Operational Experts Group (OEG) of 20 countries leads the initiative's planning and implementation of the multilateral efforts designed to further PSI's purpose. Ninety-one countries, including the OEG members, make up this group of widely multilateral participants, all of which have endorsed PSI's purpose and principles. This RAND project had two objectives. The first was to assess the advantages and disadvantages, or benefits and costs, that have induced five key countries not to affiliate with PSI publicly, and to ascertain whether this balance might be altered to enhance the prospects for their affiliation in the near future. The project's second objective was to develop a syllabus of training materials intended to help U.S. Geographic Combat Commands' ability to provide operational support for the numerous multilateral exercises constituting the core of PSI's peacetime activities. The first objective is addressed in this monograph. The five countries of interest -- Indonesia, Malaysia, Pakistan, India, and China -- share an implicit calculus that the costs (disadvantages) associated with PSI affiliation exceed, or at least equal, the benefits (advantages). The authors identify specific issues within these countries' assessments for which the benefits ascribed to PSI may have been underestimated and/or the costs ascribed to PSI may have been overestimated. They then suggest how these under- and overestimates might be changed in ways that would lead these countries to reconsider their decision not to affiliate with PSI.