Intervention is an intriguing and at times troublesome phenomenon. This book is meant for those already intrigued and those one hopes will become intrigued. The purpose is to sketch out fundamental problems that must be addressed in pursuing effective intervention. While the authors approach these problems from the perspective of psychology and education, they do so with an eclectic outlook and an emphasis on broad applications. Specifically, they identify and describe essential facets of intentional intervention, explore how these facets relate to each other, and try to produce an outline picture of the whole. The presentation is structured around the view that intervention theory and practice must address four inextricably interrelated and fundamental topics: (1) the classification problem (i.e., differentiating phenomena--conceptually and methodologically--into relevant subgroups for purposes of planning, implementing, and evaluating intervention), (2) the underlying rationale problem (i.e., assumptions shaping intervention aims and means), (3) the planning and implementation problem (i.e., processes for optimizing intervention), and (4) the evaluation problem (i.e., describing, judging, and advancing intervention knowledge and practice). Each of these is discussed in sufficient detail to facilitate development of an agenda for improving practice through theory building, program development, and research.