Online communication among patients and consumers through support groups, discussion boards, and knowledge resources is becoming more common. In this article, the summary of a workgroup discussion, we discuss key methods through which such web-based peer-to-peer communication may affect health promotion and disease prevention behavior (exchanges of information, emotional and instrumental support, and establishment of group norms and models). We also discuss several theoretical models for studying online peer communication, including social theory, health communication models, and health behavior models. Although online peer communication about health and disease is very common, research evaluating effects on health behaviors, mediators, and outcomes is still relatively sparse. We suggest that future research in this field should include formative evaluation and studies of effects on mediators of behavior change, behaviors, and outcomes. It also will be important to examine spontaneously emerging peer communication efforts to see how they can be integrated with theory-based efforts initiated by researchers.