Types of evaluation research include: (1) Formative (exploratory research), which provides information to guide planning, development, or implementation of a specific program; is always prospective; and includes needs assessments; (2) Process, which examines whether or not the services which should have been provided, were provided; can be prospective or retrospective; and (3) Outcome, which explores the effect of the program on the participants, on society, or on others; can be prospective or retrospective; includes evaluation of goals achievement, cost-effectiveness or cost-benefit analysis, and impact evaluation. The methods used in Outcome Research are True Experimental Designs including randomized control trial and cross-over design and Quasi-Experimental Designs, which include cohort studies, prospective survey, and before-after design. The major strengths and weaknesses of both methods are discussed, as are problems encountered in treatment outcome studies. Recommendations for evaluation studies are listed.