Objective: The authors investigated the longitudinal impact of peer health education on the health behaviors of undergraduate students pertaining to alcohol and drug use, eating and nutrition, and sexual health. Participants: From fall 2003 to spring 2006, the authors annually administered a Web-based survey to a cohort of 2,000 randomly selected undergraduate students. Results: Students who had contact with peer health educators were significantly more likely over time than other students to report less alcohol consumption and fewer related negative consequences. Results also show that students who had contact with peer health educators initially endorsed engaging in unhealthy weight management and fat talk behaviors during their first year but decreased these unhealthy behaviors by their third year. No significant differences in sexual health behaviors were found. Conclusions: Results indicate that peer health educators play an important role in promoting healthy behaviors in the areas of alcohol and drug use and in eating and nutrition. (Contains 2 tables and 5 figures.)