Successful replication of the AIDS retrovirus, HIV, requires that its genomic RNA be packaged in assembling virus particles with high fidelity. However, cellular mRNAs can also be packaged under some conditions. Viral RNA (vRNA) contains a 'packaging signal' (ψ) and is packaged as a dimer, with two vRNA monomers joined by a limited number of base pairs. It has two conformers, only one of which is capable of dimerization and packaging. Recent years have seen important progress on the 3D structure of dimeric ψ. Gag, the protein that assembles into the virus particle, interacts specifically with ψ, but this is obscured under physiological conditions by its high nonspecific affinity for any RNA. New results suggest that vRNA is selected for packaging because ψ nucleates assembly more efficiently than other RNAs.