Genome encapsidation is an essential step in the life-cycle of viruses. In the case of single-stranded (ss)RNA viruses, this step occurs in parallel with genome packaging in a co-assembly process. Long ssRNAs exist as multiple conformers in equilibrium, while only one of these conformers includes a compacted state allowing the RNA to be confined within the virion. We have used fluorescence correlation spectroscopy to monitor the conformations of viral genomes in the absence and presence of viral coat proteins and demonstrate that efficient selection of cognate viral ssRNA destined for genome packaging is controlled by multiple RNA-protein contacts formed during encapsidation. I will then cover mechanisms of genome encapsidation in viruses with dsRNA genomes, discussing the roles of multiple protein-RNA and RNA-RNA interactions in the assortment and packaging of segmented RNA genomes in these viruses.