Peers have intervened to promote health since ancient times, yet few attempts have been made to describe theoretically their role and their interventions. After a brief overview of the history and variety of peer-based health interventions, a 4-part definition of peer interveners is presented here with a consideration of the dimensions of their involvement in health promotion. Then, a 2-step process is proposed as a means of conceptualizing peer interventions to promote health. Step 1 involves establishing a theoretical framework for the intervention’s main focus (i.e., education, social support, social norms, self-efficacy, and patient advocacy), and Step 2 involves identifying a theory that justifies the use of peers and might explain their impact. As examples, the following might be referred to: theoretical perspectives from the mutual support group and self-help literature, social cognitive and social learning theories, the social support literature, social comparison theory, social network approaches, and empowerment models.