There is increasing evidence that the use of antiretroviral agents (ARVs) can be a safe and effective means of preventing HIV infection. In fact, a combination of ARVs, tenofovir-emtricitabine, was recently approved by the US Food and Drug Administration (FDA) for use as "pre-exposure prophylaxis"(PrEP), and the US Centers for Disease Control and other regulatory authorities have issued guidance concerning PrEP use. Clinicians and policy makers are now faced with questions about the appropriateness of prescribing ARVs to healthy persons who are at risk of becoming infected with HIV, and those at risk of being infected must decide whether to use PrEP. In addition, researcher stakeholders must grapple with determining whether and how PrEP should be included in future HIV prevention research. In addressing such issues, it is important that their ethical dimensions are identified. When using PrEP, 2 broad ethical domains are of special relevance: well-being and justice. Ethical issues related to well-being include safety, parameters of use, risk behaviors, resistance, stigma, and diversion. Those related to justice include access and competing priorities. In research involving PrEP, ethical issues include determining the appropriate control arm and whether PrEP should be included as a part of the prevention package provided to all at risk participants. Although PrEP could play an important role in HIV prevention, understanding and addressing the related ethical issues is critical to its safe, effective, and appropriate use in practice and future research.