The study of the association between therapist competence and patient improvement has provided inconsistent results (Crits-Christoph, Gibbons, & D, 2013; Luborsky, McLellan, Woody, O'Brien, & Auerbach, 1985; Sandell, 1985). It has been claimed that these findings can be partly explained by the lack of an appropriate operationalisation of therapist competence (Barber, Sharpless, Klostermann, & McCarthy, 2007). This dissertation reports on a study that replicates previous results which demonstrates no significant association between treatment fidelity and outcome, and the development of the Therapist Competence Scale (TCS), which aims to provide an appropriate operationalisation of competence in Dynamic Interpersonal Therapy (DIT). The TCS was derived from a thematic analysis of consultations with experts, and includes items on competence, incompetence, and global competence, as well as a measure of patient complexity. Data for this dissertation were based on a randomised controlled trial (REDIT study) (Lemma, Target, & Fonagy, 2011b). The reliability of the TCS was studied employing Classical Test Theory and Generalizability Theory. Validity was studied through non-parametric correlations, and analyses derived from a Confirmatory Factor Analysis. The association between competence, -as operationalised by the TCS-, and treatment outcome, was studied using multilevel modelling, in which sessions were nested within patients, and patients were nested within therapists. The results provided initial evidence for the reliability and validity of the TCS. Competence was associated with patient improvement (e.g., effectiveness), as well as with a faster rate of recovery (e.g., efficiency). Additionally, a quadratic relationship was found between competence at the level of individual sessions and outcome. Finally, a significant interaction between therapist competence and patient complexity was found, suggesting that competent therapists achieve better outcomes when treating difficult patients. With further research, the TCS could contribute to elucidating the psychotherapeutic mechanisms of change, and may potentially inform standards of psychotherapeutic training and professional practice.