Thought-listing procedures were used to examine the perceived incidence, size, direction, and bases of change in the session-level self-efficacy of therapists in training. Ninety-eight Master's-level trainees completed a cognitive assessment task immediately after each session with a client in their first practicum. Participants typically reported modest-sized, positive changes in their therapeutic self-efficacy at each session. Seven perceived sources of change in self-efficacy were identified. Some of these sources (e.g., trainees' performance evaluations, affective reactions) were consistent with general self-efficacy theory; others reflected the interpersonal performance context of therapy (e.g., perceptions of the therapeutic relationship and client behavior). Implications of the findings for training and future research on therapist development are considered. (PsycINFO Database Record (c) 2010 APA, all rights reserved).