The authors and associates conducted a social norms-based intervention targeting high-risk sexual behaviors among undergraduate students at 4 college campuses. Social norms theory predicts that widely held misperceptions may encourage risky behavior in a misguided attempt to conform to perceived norms and that information correcting these misperceptions will lead to a decrease in such behaviors. Students overestimated their peers' levels of sexual activity, numbers of partners, incidence of sexually transmitted infections, and rates of unintended pregnancies, but underestimated rates of condom use. Rates of HIV test taking, however, were accurately estimated. Although some components of sexual risk behaviors lend themselves well to social norms-based interventions, others, specifically inconsistent condom use and avoiding HIV tests, do not. Although no changes in reported beliefs or practices were apparent at the end of a 9-month intervention period, longer or modified interventions may be needed to make a fair assessment of the efficacy of this approach.