After a century of development, group therapy is today one of the most widely practised treatment methods in psychiatry with an extensive literature. There are three principles common to its wide range of applications. First, the therapist calls the ‘community’ into the consulting room where, together with the therapist, it becomes the therapeutic agent. Second, the therapist assembles a group of people who can contribute to a commonly held resource from which its members can each derive benefits. And third, the therapist does nothing for them in the context of the group, that they can do for themselves, and one another. This chapter starts by providing a conceptual framework that differentiates methods, models, and applications for the practice of group therapy in adult psychiatry. After classification of the different methods and applications we discuss the main theoretical models; explore the dynamic life of therapy groups; consider some of the key clinical issues facing practitioners; their applications to a range of patient populations and settings; their evaluation and justification and their historical evolution this century. In the conclusion we consider the planning of group services and the training of their practitioners. This revision of the chapter has brought it up-to-date with the contemporary literature in a field that has seen a great deal of innovation since the original 2000 edition. The developing evidence base for group psychotherapy is ‘Guardedly optimistic. The literature has become stronger and deeper and is capable of supporting evidence-based treatment recommendations for some patient populations.’ The evidence base for the effectiveness of group psychotherapy has been growing with the field. Some 700 studies, spanning the past three decades, have shown that the group format consistently produced positive effects with diverse disorders and treatment models. These show that both individual and group psychotherapy will effect much the same set of results. For group therapy to be effective it has to utilize those therapeutic factors originally laid out by Foulkes and later by Yalom—the group has to be the primary focus of therapy; patients need to be well selected; and therapists need to be adequately trained. The chapter will address these questions of focus, selection, and training. Although the two authors of this chapter are both group analysts, we have set out to provide a full account of the wide range of group work practice. The United Kingdom is our own working location which lends emphasis to the chapter but it is compiled with sources and references that address the international field and it gives attention to current literature in many countries including North and South America and Continental Europe.