HIV Gag assembly is the first and most essential step in the formation of virus particles. Following protein synthesis, Gag relocates from ribosomes and forms a virus particle at the plasma membrane, using host factors and machinery. Early studies focused on mapping the regions within Gag required for assembly and identified three distinct domains (M, I, and L), although their precise locations within the three-dimensional structure of Gag awaited later study. In this review, I summarize the mapping results in the light of recent progress on Gag structures made by nuclear magnetic resonance and X-ray crystallography as well as further functional analysis. These data are largely consistent and provide sufficient information for an understanding of the interactions and functions of the assembly domains at a macromolecular level. Current studies have moved on to the identification of the host factors and machinery used in the process of Gag assembly. Cumulative data suggest that the dynamics of Gag assembly and transport are achieved not by simply using, but rather by taking control of, cellular machinery. Key area in the process include interactions with TSG101, L domain receptor which normally functions in the endosomal sorting pathway and with lipid rafts, a type of M domain receptor, which has been suggested to be the sites for effective concentration of Gag. The review provides a summary of these data and discusses the likely direction of future studies.