I n the early stages of becoming a therapist, psychotherapy training and clinical supervision are deemed crucial Sharkin & Plageman, 2003). However, the development of psychotherapists does not end when therapists complete their graduate training or when they gain licensure. The great majority of clinical practice will actually take place after licensure. After a graduate training of 2 to 6 years, it is not atypical for therapists to see clients until they retire 40 to 70 years later. Arguably, professional development begins in graduate training, but being a competent therapist is a lifelong learning experience .The primary goals of this chapter are (a) to provide an overview of empirical research on the effect of professional training and supervision of experienced therapists and (b) to offer suggestions for clinical and research questions that should be examined in the future. In terms of the scope of the review, we consider all types of professional development endeavors that licensed therapists might engage in, including in-person training, such as conferences and workshops as well as supervision and personal therapy. Following the pandemic, research on online formats of professional training and supervision has gained a greater momentum and thus is included in this chapter. Notably, the research on deliberate practice is also relevant to the training of licensed