This chapter presents an overview of psychotherapy research. It starts by setting out a set of principles supported by scientific evidence and clinical experience. These principles work as synthesis of the main aspects the chapter aims to highlight about psychotherapy research. The differences between the medical and the contextual model are then presented. The chapter falls into three main parts: (a) summary of different aspects emphasized by research, such as the general efficacy of different therapeutic models ; the therapeutic alliance and the repair of therapeutic ruptures ; the common factors and the specific techniques ; expectations , goals and level of collaboration between the client and the therapist ; (b) feedback systems in psychotherapy; and (c) therapist factors .