This chapter focuses on the characterization, classification, and taxonomy of viruses that infect humans, both pathogens that infect humans alone and those that infect humans but may also infect other mammalian or nonmammalian genera and species. While other forms of life encode genetic information within double‐stranded DNA (dsDNA), viruses encode their genetic information within genomes that may be composed of single‐stranded RNA, double‐stranded RNA, single‐stranded DNA, and dsDNA. The Baltimore classification, a nonhierarchical approach, categorizes viruses into seven groups and is based on the genome present in virions and type of replication. Classification of viruses below the level of species is not standardized across all species by the International Committee on Taxonomy of Viruses, but in some cases, subspecies standardization does occur. Between 2016 and 2021, a series of changes have occurred in the taxonomy of viruses that infect humans.