Treatment fidelity concerns whether therapy is implemented as intended and an important aspect of treatment fidelity is therapist competence. Therapist competence scales are a popular method of assessing whether therapy is competently delivered, however these scales vary in their development and evaluation, and in their resulting reliability and validity. This thesis presents a review of therapist competence scales and an empirical study that investigated a new therapist competence scale for Compassion Focused Therapy. Systematic Literature Review: The systematic literature review explored the development and psychometric evaluation of therapist competence scales for cognitive and cognitive-behavioural therapies. Four databases were searched, and thirteen papers were included in the review. The standard of papers was assessed using a bespoke quality appraisal tool and results were narratively synthesised. There was little consensus about the methods that should be used to develop and evaluate competence scales and it was concluded that researchers might use multiple methods of assessing competence rather than rely on competence scales alone. Empirical Research Project: A mixed-methods approach was used to explore the psychometric properties and expert feedback of the Compassion-Focused Therapy Therapist Rating Scale (CFT-TCRS; Horwood et al., 2019). CFT experts watched clips of simulated CFT and used the CFT-TCRS to assign competence ratings before engaging in a semi-structured interview. Inter-rater reliability between participants was ‘good'. Content analysis of the expert feedback provided a number of useful suggestions to improve the scale. Once amended the CFT-TCRS may become a useful tool for clinical practice, therapist training and continued research into aspects of treatment fidelity in CFT.