Group therapy is widely used as a primary treatment modality for treating adult sex offenders. There is a rich history of group therapy research and literature on the therapeutic effects of treatment in a group yet there has been little research on the therapeutic benefits of group based interventions for sex offenders. This chapter presents a brief history of group therapy, summarizes the literature, and discusses effective group characteristics. Assessment, ethics, and therapist issues are also discussed. The chapter argues that sex offender treatment can be made more effective when the group is used to its full potential. (PsycINFO Database Record (c) 2018 APA, all rights reserved)