The diagnosis and psychosocial treatment of moderately head injured clients requires three elements. First, there should be a stated frame of reference for the understanding of client behavior within the constraints of the injury. Second, and related to the first, there should be at work a practice theory that is consistent with the therapist's view of human behavior. Third, interventions must be consistent with both. In this article, the authors describe and discuss all three needed elements. The membership perspective serves as the practice theory and as the foundation for the operationalization of group therapy with the head injured client.