The application of self-determination theory (SDT) to psychotherapy is particularly relevant because a central task of therapy is to support the client to autonomously explore, identify, initiate, and sustain a process of change. In this article, the authors discuss the experimental work, field studies, and clinical trials representing the application of SDT to the domain of psychotherapy. Evidence supports the importance of client autonomy for the attainment and maintenance of treatment outcomes. In addition, intervention studies suggest that therapist autonomy support enhances the likelihood that treatment gains will be achieved and maintained. The authors discuss some of the processes involved in enhancing autonomy, including the role of awareness, the importance of exploring and challenging introjects and external regulations, attention to need-related goal contents, and therapist attitudes required for a therapy approach that is process- rather than outcome-focused.