Gag proteins are synthesized and transported to the PM. Many studies demonstrate that the major site of HIV-1 assembly is the PM [15] [16] [17] [18] , although late endosomes could be a platform for virus assembly under specific conditions [19] . In primary macrophages, HIV-1 has been shown to assemble in endosomal vesicles. However, studies have recently suggested that the above vesicles are not late endosomes but rather membrane invaginations connected to the PM [20] [21] [22] . In addition to Gag, the other major structural retroviral protein is the Env glycoprotein. Env proteins are required for virus entry into target cells and are thus essential for forming infectious retroviral particles. In this paper, we discuss current knowledge about the biosynthesis, intracellular trafficking, and virion incorporation of retroviral Env proteins, as well as the membrane microdomains involved in virus assembly and/or transfer. Most of this information was obtained from studies on HIV-1.