Applied behavior analysis (ABA) differs from other areas of applied psychology in many respects, but two are especially prominent. First, ABA is not an eclectic enterprise, borrowing theory and method from varied persuasions; it is grounded in the theoretical and experimental orientation of behavior analysis. Second, whereas most applied fields are distinguished by their emphasis on a particular clientele, problem, or setting, ABA is constrained only by its principles and methods. ABA focuses on any aspect of human and sometimes nonhuman behavior, regardless of who emits it or where it occurs, crossing professional boundaries typically used to define clinical, educational, and organizational psychology as well as generational cohorts and diagnostic categories. Thus, the subject matter of ABA is not tied to any specific area of application. Other chapters in this handbook present summaries of applied research organized along more traditional lines. In this chapter, we emphasize ABA's distinctive features and summarize its major themes on the basis of the behavioral processes of interest.