The quality control of therapy in routine clinical practice depends to a large degree on the ability of therapists to evaluate accurately their own performance in administering therapy. However, the literature in many fields casts doubt on the accuracy of people's self-evaluations. This study aimed to examine the accuracy of therapists' judgments about their own competence in cognitive therapy. Twenty-two therapists rated a tape of one of their cognitive therapy sessions from the middle of therapy using the Cognitive Therapy Scale (CTS) and provided information about their profession and their training in cognitive therapy. An independent expert rater, blind to all information about the therapist, also rated these tapes on the CTS. Therapists were coded as Competent or Less Competent on the basis of the observer-rated CTS score. Whilst there was a significant correlation between self-ratings and expert ratings of competence, therapists significantly over-rated their competence relative to the expert rater. Less competent therapists over-rated their own competence to a greater degree than therapists who met criteria for competence. The finding that therapists, especially less competent therapists, over-rate their competence in cognitive therapy has serious implications for ensuring effective practice of cognitive therapy in routine clinical situations.