This paper reviews the literature on the developmental stages of trainees and supervisors and how experience changes each of these groups. The assumption that trainees and supervisors learn and improve as they gain in experience does not explain how psychology trainees and supervisors change. The nature of this change leads to a number of questions: (1) Should the supervisory process be proactive or reactive? (2) Should the trainee learn the theoretical orientation of the supervisor or should the supervisor adapt to the trainee's theoretical orientation? and (3) Should supervision models be based upon counseling theories? While much of the research on trainee changes implies a stage theory of trainee development, there has been little systematic research on supervisor changes. What is known about supervisor development reveals that supervisors do not become more competent as they gain experience. Since practicums are essential in trainee development and since supervisors of trainees have to make every minute of supervision count, a co-therapy or co-intake procedure allows the supervisor to offer trainees more immediate and relevant feedback regarding intake and therapy sessions. Co-therapy also immerses trainees in the therapy process, allowing a continual focus on the student client and on the trainee's needs. (RJM)