More than 25 years into the HIV epidemic, in excess of 2 million new infections continue to occur each year. HIV prevention research is crucial for groups at heightened risk for HIV, but the design and conduct of HIV prevention research with vulnerable populations worldwide raises considerable ethical challenges. The HIV Prevention Trials Network (HPTN) is a global collaborative network that conducts clinical and behavioural studies on non-vaccine interventions to reduce the transmission of HIV. In 2003, the HPTN developed ethical guidance to enhance the responsible conduct of its research activities and as a distinctive contribution to global research ethics. In what follows, the developments that motivated the drafting of a revised ethics document in 2009 are described, including the process by which that revision took place and some of the key differences between the HPTN ethics guidance and other relevant guidelines in the field.