In the current paper, we attempt to show how both the basic and applied sciences of behavior analysis have been transformed by the modern research agenda in human language and cognition, known as Relational Frame Theory (RFT). At the level of basic process, the paper argues that the burgeoning literature on derived stimulus relations calls for a reinterpretation of complex human behavior that extends beyond a purely contingencybased analysis. Specifically, the paper aims to show how a more complete account of complex human behavior includes an analysis of relational frames, relational networks, relating relations, rules, perspective-taking, and the concept of self. According to the theory, this analysis gives rise to a new interpretation of human psychopathology that necessarily transforms the applied science of behavior therapy. The current paper is divided into three parts. In Part 1, we provide a brief summary of the integrated history of behavioral psychology and behavior therapy, including their emphases on the principles of classical and operant conditioning as the basis for an account of human psychopathology. In Part 2, the core features of RFT are presented, including the three concepts of bidirectional stimulus relations, relating relations, and rule-governance that constitute critical components of the RFT approach to human psychopathology. The paper therein attempts to illustrate, with the use of clinically relevant examples, the ways in which these concepts can be used to understand psychopathology and psychotherapy. In Part 3, RFT interpretations of three central features of Acceptance and Commitment Therapy (ACT), namely acceptance, defusion, and values are provided with a view to demonstrating the utility of basic RFT concepts in the treatment of human suffering.