Definitions and short explanations of key concepts in the field of instructional evaluation are presented. Evaluation is defined as the process of determining the value of programs, projects, materials, and personnel. It is distinguished from research in that the aims of research are less time- and situation-specific, attempting to uncover principles that apply universally. In evaluation, less attention is paid to generalizing findings to a larger population. Astute evaluators argue that thorough evaluation begins as the program is conceptualized and planned. Also important is the distinction between formative evaluation conducted during the development or improvement of a program or product, and summative evaluation conducted after completion of an effort. Considerable attention is being given to the balance between quantitative and qualitative measures as part of an evaluation. The two most important criteria for judging evaluation seem to be isomorphism (similar or identical structure so as to fit with reality-based information) and credibility (believability). The two most important criteria for judging the adequacy of research are internal validity and external validity. Evaluation is the disciplined inquiry that yields information about the performance of educational programs, products, and processes. One figure pictures the inquiry domain. (Contains 7 references.) (SLD)