Program managers and policy makers need to balance the costs and benefits of various interventions when planning and evaluating HIV prevention programs. Resources to fund these programs are limited and must be used judiciously to maximize the number of HIV infections averted. Economic evaluation studies of HIV prevention interventions, which we review and critique here, can provide some of the needed information. Special emphasis is given to studies dealing with interventions to reduce or avoid HIV-related risk behaviors. Ninety-three cost-benefit, cost-effectiveness and cost-utility analyses were identified overall. However, only 28 dealt with domestic, behavior change interventions; the remainder focused on screening and testing without prevention counseling, and on care and treatment services. There are compelling demonstrations that behavioral interventions can be cost-effective and even cost-saving. The threshold conditions under which these programs can be considered cost-effective or cost-saving are well defined. However, several important intervention types and multiple key populations have been unstudied. Research in these areas is urgently needed.