This paper describes the initial evaluation of the Therapist-Parent Interaction Coding System (TPICS), a measure of in vivo therapist coaching for the evidence-based behavioral parent training intervention, parent-child interaction therapy (PCIT). Sixty-one video-recorded treatment sessions were coded with the TPICS to investigate (1) the variety of coaching techniques PCIT therapists use in the early stage of treatment, (2) whether parent skill-level guides a therapist's coaching style and frequency, and (3) whether coaching mediates changes in parents' skill levels from one session to the next. Results found that the TPICS captured a range of coaching techniques, and that parent skill-level prior to coaching did relate to therapists' use of in vivo feedback. Therapists' responsive coaching (e.g., praise to parents) was a partial mediator of change in parenting behavior from one session to the next for specific child-centered parenting skills; whereas directive coaching (e.g., modeling) did not relate to change. The TPICS demonstrates promise as a measure of coaching during PCIT with good reliability scores and initial evidence of construct validity.