A review of the dimensions of evaluation addresses program objectives, design, and implementation; defining and measuring outcomes; the value of controlled experiments; and constraints on evaluation. The chapter recommends the expanded use of randomized field experiments to evaluate new intervention programs on both individual and community levels. The randomized field experiment requires that individuals, organizations, or other treatment units be randomly assigned to one of two or more treatments or program variations. In discussing special concerns of evaluation in the context of HIV infection and AIDS, the chapter discusses target groups, the information base, the objective of AIDS intervention programs, ethical issues, attrition and noncompliance, and measurement and observation. The chapter offers recommendations for implementing effective evaluations of AIDS interventions. Among the recommendations are that evaluation support be provided to ensure collaboration between practitioners and evaluation researchers, that only the best-designed and best-implemented intervention programs be selected to receive special resources for scientific evaluations, and that the results of well-conducted evaluations be published regardless of the intervention's effectiveness. 140-item bibliography.