Functional Analytic Psychotherapy (FAP) is a talk therapy based on the principles of behaviourism that was developed by Robert Kohlenberg and Mavis Tsai. Kohlenberg and Tsai noted that rapid therapeutic gains frequently occurred in therapeutic contexts that were genuine, emotionally intense, and characterised by interpersonal intimacy. The developers of FAP detail how through functional analysis, therapists can work with clients to collaboratively identify the contingencies maintaining these behaviours and apply differential reinforcement to promote meaningful change while simultaneously strengthening the therapeutic relationship. This chapter traces the historical development of FAP and elaborates on its theoretical underpinnings. Emphasising moment-to-moment interactions within the therapeutic relationship, FAP offers a replicable, empirically grounded framework for promoting behavioural transformation. By integrating radical behaviourism’s expansive perspective on both observable and private events, FAP advances the understanding and treatment of complex human behaviours. Review of the history of FAP has indicated multiple advancements and future directions of the treatment, which are discussed and underscore FAP’s role in the broader landscape of contextual behavioural science informed therapies.