The recent NIH HPTN 052 study of using HIV treatment to prevent HIV transmission in serostatus discordant heterosexual partnerships has garnered much attention. In subsequent discussions, however, the topic of HIV-related risk behavior has been nearly absent. Here, we identify the critical roles that HIV-related risk behavior plays in determining the unmet needs, optimal targeting, and ultimate impact of treatment as prevention. We describe the size of the population at risk of HIV and three subgroups of persons living with HIV (PLWH) based on awareness of serostatus and risk behavior, and the corresponding HIV transmission rates to seronegative partners. For each of the subgroups of PLWH, we identify which approach is most relevant ("testing and linkage to care," "treatment as prevention," and/or "treatment as clinical care"). We observe that the impact of "treatment as prevention" on HIV incidence will depend heavily on which subgroup of PLWH is targeted for services.