Human immunodeficiency virus (HIV) infection leads to a prolonged struggle between a rapidly evolving viral population and a potent immune response. In the vast majority of infected individuals, the virus wins this struggle. In my laboratory, we focus on understanding both the viral and immune factors that contribute to this outcome. The results of our studies and those of many others indicate that HIV can escape a potent immune response by a combination of mechanisms including rapid mutation, shedding of decoy antigens, modulation of host major histocompatibility complex, and destruction of cytotoxic T lymphocytes. The target cells for viral infection change as the virus evolves to use different chemokine coreceptors for entry. The initial targets are activated and resting memory T cells that express both CD4 and CCR5, but both naive and memory CD4 T cells are targeted by viruses capable of using CXCR4 for entry, and macrophages become the primary target cells when most CD4 T cells are depleted. Compelling evidence is emerging that the availability of target cells for infection is as limiting for the spread of virus as the immune response.