HIV-1 particle assembly is driven by oligomerization of the Gag polyprotein precursor in infected cells. Translation of the full-length viral RNA results in the synthesis of large amounts of Gag molecules which oligomerize upon the combined interactions of its C-terminal NC domain with the genomic RNA and of its N-terminal myristate and matrix basic residues with cellular membranes. HIV assembly has been studied during the past two decades mostly by means of biochemical techniques on transfected or infected cells as well as in vitro using purified Gag molecules. More recently, understanding the mechanisms of viral assembly moved a big step forward due to the utilization of fluorescently labeled viral proteins, notably Gag, and of fluorescent microscopy techniques able to track single viral particles. In this chapter, we will summarize recent imaging data on HIV-1 Gag assembly at the level of the plasma membrane where viral particles bud in the form of cell-free viruses or can be transmitted to adjacent naïve cells through virological synapses.