We introduce interpersonal behavior therapy (IBT) in the context of a brief history of evolving paradigms of psychotherapy research and the rise of the third-wave behavior therapies facing the challenges of the introduction of middle-level terms in the service of their dissemination. The article focuses on IBT as a response to the evolution of functional analytic psychotherapy (FAP) and its movement away from behavioral principles and functional assessment. IBT is proposed as a contemporary behavioral treatment whose focus is on interpersonal distress, emphasizes the need for a functional assessment to conceptualize client problems, and utilizes behavioral principles to specify the mechanisms of the problem and mechanisms of clinical change. Largely a retooling of the original proposals in FAP, IBT explicates the mechanism responsible for clinical problems and the corresponding mechanism of clinical change. Moreover, as a behavioral therapy, IBT emphasizes the need for functional assessment in conceptualizing client problems and determining clinical treatments. Finally, we call for a unified return to behavioral assessment across the third-wave therapies. This unified approach may help advance principle-driven treatments for complex forms of human suffering as well as offer a path forward to a program of behavioral science and preserve the longevity of behavioral therapies.