This research aimed to systematically examine supervisor-trainee differences in assessments of trainee competencies across domains and developmental stages. Trainees and supervisors (N = 141 dyads) independently rated trainee performance at the end of placements using the Clinical Psychology Competencies Rating Scale. Based on the number of placement hours completed at the time competence was assessed, the 141 trainees were assigned to three developmental levels (61, 42, and 31 in the groups, respectively). Trajectories of 10 different competencies and trainee-supervisor differences for these competencies were examined across three developmental levels. Compared to their supervisor ratings, trainees underestimated their competence during early stages of training, with this discrepancy reducing at Level 2 and reversing into an overestimation at Level 3. Compared to their own ratings for overall competence, trainees rated Relational and Communication,Reflective Practice, and Professionalism domains as relative strengths, and rated their competence on assessment and intervention domains as relative weaknesses. Growth trajectories derived from supervisor assessments were much flatter than trajectories derived from trainee assessments. As predicted by the impostor theory of practitioner development, trainees significantly underestimated their competence early in training. The trend for trainees to overestimate their competence towardthe end of their training is a potential concern that warrants further research.