Engraftment of immunodeficient mice with a human immune system (humanized mice) provides a model system to study pathogens that target human immune cells. Humanized Rag2−/− γc−/− mice produce the major target cells of HIV-1 and these cells can be detected in primary and secondary lymphoid tissues, as well as in the vaginal and rectal mucosa and brain tissues. This humanized model has already yielded important findings on HIV-1 transmission, mechanisms of pathogenesis, and testing of novel antiviral strategies in vivo. Here, we describe the methods used to infect humanized mice with HIV-1 and to characterize plasma viral load and blood CD4+ T cell depletion.