Therapist effects have been increasingly recognized as an important contributor of psychotherapy process and outcome. Most therapist factors studied so far, however, have been trait factors. Little is known about state factors. Given the emotional nature of psychotherapy, therapist affective states seem relevant. In particular, how does therapist affect change in sessions? What predict therapist affect change, and how is therapist affect related to psychotherapy process and outcome? Data involved 1,172 sessions of 15 therapists and 51 clients at a psychodynamically-oriented psychotherapy clinic. Therapists and clients rated pre-session affect and post-session affect, as well as post-session working alliance, session quality, and real relationship. Participants also wrote down their affect changes, and attributions to these changes, at the end of each session. Quantitative data were analyzed using multilevel modeling. Qualitative data were analyzed using Consensual Qualitative Research. Therapists qualitatively reported affect changes in 67% of sessions, with equal amounts of increases in positive and negative affect. Therapists most frequently attributed their increase in positive affect to being able to collaborate with clients, and their increase in negative affect to having difficult clients. Therapist pre- to post-session change in affect was related to client pre-session affect and client pre- to post-session change in affect. After controlling for therapist change in affect from pre- to post-session, higher therapist pre-session positive affect was associated with better client-rated working alliance and session quality, whereas higher therapist pre-session negative affect was associated with poorer client-rated session quality. Increase in therapist positive affect from pre- to post-session was related to better client-rated session quality and therapist-rated working alliance, session quality, and real relationship, whereas increase in therapist negative affect was related to poorer client-rated real relationship and therapist-rated working alliance, session quality, and real relationship. Thus, therapist affect played a role in therapist functioning and contributed to psychotherapy process and outcome.