Therapist competence refers to the extent that a given treatment is conducted in accordance with the instructions or intentions of the respective treatment manual. Despite this relatively straightforward notion, existing research on cognitive-behavioural therapy (CBT) processes have been unsuccessful in defining and measuring this construct. This article reviews the contemporary empirical research on therapist competence in CBT, outlines the development and psychometric evaluation of the commonly used measures of therapist adherence, and discusses how competence has been linked to treatment outcomes. The psychometric evidence for existing measures is mixed, and in particular, there has been difficulty in the demonstration of adequate interrater reliability, even among identified experts in the field. New measures of therapist competence hold promise — most notably, in the separation of therapist adherence and competence constructs. The assessment of therapist adherence, therapist competence, and the role of supervisor ratings in the clinical context are also discussed.