Psychology is defined as “the study of mind and behavior” (American Psychological Association, n.d., How Does the APA Define “Psychology”? section) where mind is the subject matter and behavior is what is studied to develop theories of mind. Thus, most psychological researchers either explicitly or implicitly infer hypothetical mental or cognitive structures and processes solely from observed behavior. One branch of psychology—behavior analysis—has challenged this orthodoxy, though with only marginal success. Both the basic science—the experimental analysis of behavior—and the philosophy of behavior analysis—radical behaviorism—have been misunderstood and misrepresented for decades. In this article, I describe how the basic science and philosophy of behavior analysis, as first articulated by B. F. Skinner, has represented the most heterodox view in psychology. I discuss several ways that behavior analysis differs dramatically from most of mainstream psychology. First, the subject matter of behavior analysis is behavior, specifically functional relations between actions and environmental controlling variables. Second, behavior-analytic theory is selectionist on the analogy of natural selection in biology. Third, experimentation is analytical wherein independent variables are manipulated within subjects to discover orderly relations between those variables and behavior. Fourth, behavior-analytic theory appeals to observed, or potentially observable, physical events, and not hypothetical (mental or cognitive) constructs, as explanations of behavior, and, as a result is patently monistic. Thus, the role traditionally assigned to mental events is played, in large part, by covert behavior. These characteristics of behavior analysis place it squarely in the natural science camp and, hence represent a heterodoxy within psychology. (PsycINFO Database Record (c) 2018 APA, all rights reserved)