||Virus budding is a fission event in which the continuous cell membrane connecting a virion to the cell is severed, creating discrete viral and cellular membranes. Enveloped viruses do not encode their own budding machinery, and must recruit cellular membrane fission proteins to assist in the process. This thesis describes research aimed at understanding how cellular factors interact with the structural proteins domain of Human Immunodeficiency Virus (HIV) to facilitate virus budding. A short sequence motif found in the p6 domain of HIV-1 Gag (PTAP, the "late" domain) is necessary for efficient viral release from cells. A series of distinct, but functionally interchangeable late domains have been described in other enveloped viruses, suggesting that viral budding may proceed via a common, host factor-mediated pathway. We and others have identified the cellular protein Tsg101 as the functional receptor for the HIV-1 late domain. Tsg101 normally functions in vacuolar protein sorting, where it helps sort membrane-bound cargo for incorporation into vesicles that bud into the maturing endosome to create multivesicular bodies (MVBs). MVB formation is topologically equivalent to virus particle formation at the plasma membrane, and the same machinery could, in principle, catalyze both processes. I have used RNA interference (RNAi) in cultured human cells to formally establish that Tsg101 is required for release of infectious HIV-1 virions. Depletion of endogenous Tsg101 severely attenuates HIV-1 budding at a very late stage, and introduction of RNAi-resistant Tsg101 into these cells rescues virus budding. Furthermore, the regions of Tsg101 that are responsible for PTAP-binding and Vps28-binding are essential for budding, thus emphasizing the importance of these interactions for viral egress. Interestingly, depletion of Tsg101 from cells does not inhibit Moloney murine leukemia virus (M-MLV, PPXY late domain) budding. Nevertheless, functional vacuolar protein sorting machinery is required for late domain mediated budding of retroviruses as overexpression of dominant negative Vps4 mutant proteins that disrupt vacuolar protein sorting and Tsg101 trafficking severely attenuate release of both HIV-1 and M-MLV. Overall, the data presented here support a model in which retroviruses, and potentially all enveloped viruses, appropriate cellular machinery used to form MVB vesicles to facilitate budding.