Book excerpt: A cutting-edge collection of basic and state-of-the-art methods optimized for investigating the molecular biology of this class of retrovirus. These readily reproducible techniques range from methods for the isolation and detection of human retroviruses to cutting-edge methods for exploring the interplay between the viruses and the host. Here, the researcher will find up-to-date techniques for the isolation and propagation of HIV, HTLV, and foamy virus from a variety of sources. There are also assays for determining the cell tropism of HIV-1, the coreceptor usage of HIV-1, and human gene expression with HIV-1 infection by microarrays, as well as for phenotyping HIV-1 infected monocytes and examining their fitness. Highlights include the detection and quantification of HIV-1 in resting CD4+, a new cloning system for making recombinent virus, cDNA microarrays, and the determination of genetic polymorphisms in two recently identified HIV-1 co-factors that are critical for HIV-1 infection.