The thesis of this article is that the lack of evidence related to the identification and development of therapist expertise is due to the inadequate definition and operationalization of the concept. We propose a definition of expertise that is restricted to performance in the conduct of psychotherapy: the manifestation of the highest levels of ability, skill, professional competence, and effectiveness. In addition, we offer several criteria that may be used to assess expertise: performance (including relational and technical expertise), cognitive processing, client outcomes, experience, personal and relational qualities, credentials, reputation, and self-assessment. We then review research related to the development of expertise, highlighting the role of experience with clients, personal therapy, supervision, deliberate practice, and feedback. Finally, we conclude with recommendations for conducting research on therapist expertise.