This paper traces the developmental history of acceptance and commitment therapy (ACT) from its beginning as comprehensive distancing to its current form and status. It is maintained that technical differences between the two approaches are overshadowed by ones of conceptualization. Comprehensive distancing emerged from efforts to extend Skinner's work on verbal behavior and rule-governance to clinical phenomena, while relational frame theory as a post-Skinnerian account of human language has served as the conceptual foundation for ACT. Possible research strategies to further clarify conceptual differences between the two approaches are discussed.