This article presents a complex case which was treated through psychotherapy. This study was undertaken in the first instance using the methodology of the French Psychotherapy Practice-Based Research Network (Réseau de Recherches Fondées sur les Pratiques Psychothérapiques), then subsequently in the course of a seminar made up of experienced clinicians. The study focuses on the process of change by highlighting the actions and attitudes of the therapist to support his patients in tackling his problems and sufferings. Several evaluations tools (qualitative and quantitative) are used. To begin with the Case Formulation, that brings to bare a fundamental clinical expertise; then two tools centred on the patient: the Health-Sickness Rating Scale and the Psychodynamic Functioning Scale; and finally a tool specific to psychotherapy, the Psychotherapy process-Q-set. The methodology put in place makes it possible to have reliable indications regarding what is taking place in the psychotherapy, and the impact it is having on the positive evolution of the patient. Discussion of the results after a year of therapy already make it possible to draw links between theory and practice on certain aspects, notably the framework, or the difficult place of the therapist in this psychotherapeutic context. This is in-depth research, and an account of the “Case Study”.