Whether it is in a mental health hostel, psychiatric ward, day hospital, or social care setting, group processes are always operating and have a profound influence on the day-to-day running of services for people with severe mental illness. This chapter will outline the three predominant theories of group psychotherapy that are used to understand how groups work, both informally and formally, and then go on to analyse clinical situations where these theories can help us understand the complexity of interactions, particularly between staff and patients.