RNA pseudoknots are structural elements found in almost all classes of RNA. First recognized in the genomes of plant viruses, they are now established as a widespread motif with diverse functions in various biological processes. This Review focuses on viral pseudoknots and their role in virus gene expression and genome replication. Although emphasis is placed on those well defined pseudoknots that are involved in unusual mechanisms of viral translational initiation and elongation, the broader roles of pseudoknots are also discussed, including comparisons with relevant cellular counterparts. The relationship between RNA pseudoknot structure and function is also addressed.