The importance of clinical supervision is increasingly recognized, as is the need to demonstrate that the work we do in the NHS is effective. However, observational analyses of supervision or evidence bearing out the effectiveness of supervision are rare and narrow in focus. To contribute to the evidence base, the present N=1 content and outcome evaluation describes and then assesses the effectiveness of cognitive behaviour therapy supervision, in terms of its observed impacts on a supervisee and her patient. The supervisee was a trainee therapist participating in a diploma course in cognitive therapy. The study utilized a qualitative and quantitative content analysis methodology, based on the intensive coding of a series of 10 longitudinal, video-recorded supervision sessions, linked to the subsequent 10 therapy sessions. Based on this method, 14 supervisory themes were extracted, which served to describe the change methods employed in the supervision. The predicted transfer (generalization) of those themes from supervision to therapy was observed to occur to a surprisingly marked extent, indicating that the supervision was effective. Implications are drawn for developing supervision and related research.