Soon after entry into their host cells ssRNA viruses form doule-stranded replicative intermediates. Most RNA viruses exist only transiently in this double-stranded form. Double-strand formation occurs irrespective of whether the genome is of the same or complementary polarity to viral mRNA. In contrast, reoviruses contain dsRNAs from the outset. Single, and not double, strands are the intracellular intermediates through which genetic information is transferred from parental to progeny reoviruses. Double strands are the repositories in which the information is stored. We emphasize this distinction because it permits dsRNA-containing viruses to replicate conservatively, a mode of replication that is not shared by any other viruses. One important consequence of the conservative mode of replication is that cellular enzymes never gain access to the reovirus genome but only to its ssRNA precursors.