The chapter outlines an evidence-based approach for clinical psychologists to select and provide group treatments. The process begins with selecting the appropriate treatment programme and then selecting the appropriate patients for the group. The chapter will identify principles for selection and assessment as well as how the use of interpersonal interaction as a therapeutic tool in the here-and-now context of a group can be an inherent advantage of group interventions.In addition, it outlines practical steps for implementation of a group that includes pre-group orientation, enlisting patients as informed allies, providing guidelines about how best to participate, clarifying the format and duration, setting ground rules, anticipating problems and instilling optimism among members. The chapter concludes with a consideration of how monitoring of process and outcomes can enhance outcomes.