Male circumcision (MC) can prevent female-to-male HIV transmission and has the potential to significantly alter HIV epidemics. The ultimate impact of MC on HIV prevention will be determined, in part, by behavioral factors. In order to fully realize the protective benefits of MC, factors related to acceptability and sexual risk must be considered. Research shows that acceptability of MC among uncircumcised men is high, and suggests that free and safe circumcision may be taken up in places with high HIV prevalence. Perceptions of adverse effects of MC may, however, limit uptake. Furthermore, considerable risk reduction counseling provided by MC trials limits our ability to understand the impact MC may have on behavior. There is also no evidence that MC protects women with HIV-positive partners or that it offers protection during anal intercourse. Research is urgently needed to better understand and manage the behavioral implications of MC for HIV prevention.