<p>This article presents radical behaviorism as the philosophical foundation of behavior analysis and argues for its practical relevance to contemporary applied behavior analysis. Written primarily for students and instructors, it is motivated by upcoming credentialing and accreditation standards that will require substantial coursework in behaviorism and philosophical foundations. The article focuses especially on explaining behavior from a radical behaviorist perspective and clarifying the assumptions that underlie the science of behavior analysis. After briefly distinguishing among the conceptual, experimental, applied, and service-delivery aspects of the field, the article traces the historical development of behaviorism from Watson and Thorndike to Skinner’s experimental work and his formulation of radical behaviorism. The article emphasizes that radical behaviorism treats behavior—public and private—as physical events subject to lawful relations, rejects mentalistic and dualistic explanations, and interprets verbal behavior and private events (e.g., thinking, consciousness) as environment behavior relations. The core assumptions of radical behaviorism—monism, empiricism, determinism, parsimony, selectionism, and pragmatism—are outlined, and their implications for theory, explanation, and practice are explained. Finally, radical behaviorism is framed as both a professional guide for behavior-analytic activity and a broader worldview that can foster compassion, improve practice, and support cultural and personal change.</p>