Viruses are obligate intracellular parasites that can infect organisms throughout the tree of life. Viruses vary greatly in physical structure, genome size and genome structure. Viral genomes can be single-stranded or double-stranded and either DNA or RNA. Accordingly, viruses have evolved diverse strategies for sabotaging host cells into replicating viral genomes, with the result often being the death of the host. The molecular mechanisms of viruses raise intriguing questions: How do viruses trick host cell defenses into permitting their entry and replication? How did viruses originate: as free-living cells that took up predatory lifestyles or as host cell components that went wrong? Viral molecular biology offers clues to these questions. The molecular mechanisms of viral infection reveal targets for antiviral drugs and tools for gene therapy vectors.