We use studies of HIV/SIV evolution to reveal information about viral dynamics in vivo. This typically involves genetic and/or phenotypic analyses of viral populations in samples from HIV-infected humans or SIV-infected nonhuman primates (NHPs). We are currently exploring the mechanisms that contribute to neurocognitive impairment in HIV-infected people by sequencing viral populations in the CNS of humans and NHPs not on antiretroviral therapy. We are also using these approaches to examine viral populations that persist during long-term antiretroviral therapy in an effort to better understand the viral reservoirs that must be targeted in order to cure HIV-infected people.