HIV-1 particle assembly is driven by the Gag polyprotein precursor, which contains several structural and functional domains that engage in protein-protein, protein-lipid, and protein-RNA interactions during virion assembly. Concomitant with virus release from an infected cell, the viral protease cleaves the Gag precursor to liberate the mature Gag proteins, triggering a morphological transformation of the virus particle, called maturation. The matrix (MA) domain plays key roles in directing Gag to the plasma membrane of the host cell and in the incorporation of the viral envelope glycoprotein (Env) complex into the assembling particle. On page 700 of this issue, Qu et al. (1) report the cryo–electron tomography (cryo-ET) structure of MA in both immature and mature particles. The results provide important insights into HIV-1 assembly and maturation and the role that MA plays in these processes. These findings may suggest new antiviral strategies that target MA.