Case study methods involve an in-depth, longitudinal examination of a single instance or event. They result in an increased understanding of why the instance happened as it did, and are especially well suited for generating, rather than testing, hypotheses. Six types of case studies are discussed, based on the framework of L. Datta (1990). For each, the type of evaluation questions that can be answered, the functions served, some design features, and some pitfalls are reviewed. Illustrative case studies are descriptive; they use one or two instances to show what a situation is like. Exploratory case studies are condensed case studies, which are undertaken before implementing a large-scale investigation. Critical instance case studies examine one or a few sites to study situations of unique interest or to test a generalized or universal assertion. Program implementation case studies help discern whether implementation is in compliance with its intent. Program effects case studies determine the impact of programs and provide some inferences about reasons for success or failure. Cumulative case studies aggregate data from several sites collected at different times. Evaluators considering the case study as an evaluation design must decide what type of question they have and examine the ability of each type of case study to answer the specific question. There are four references. (SLD)