High-throughput sequencing (HTS) and its use in recovering and assembling novel viral sequences from environmental, clinical, veterinary, and plant samples have uncovered a vast new catalog of viruses (Lefkowitz EJ et al. 2018). Their classification, known by their sequences alone, poses a major challenge to traditional virus taxonomy, particularly at the family and species levels, which have historically been largely based on descriptive taxon definitions. These typically include some knowledge of their phenotypic properties, including replication strategies, virion structure, and clinical and epidemiological features such as host range, geographic distribution, and disease outcomes. However, for viruses identified in metagenomic datasets, little to no information is available. If such viruses are to be included in virus taxonomy, their assignments must be guided largely or entirely by metrics of genetic relatedness. The immediate problem with this is that the International Committee on Taxonomy of Viruses (ICTV), an organization that approves the taxonomic classification of viruses, provides little or no guidance on how similar or how different viruses must be to be considered members of new species or new families. A rapid and objective means to explore metagenomic viral diversity and make evidence-based assignments for such viruses at each taxonomic layer is essential. Sequence analysis can provide evidence that family assignments (and genera) of currently classified viruses are largely underpinned by genomic relatedness, and these features could guide evidence-based classification of metagenomic viruses in the future (Simmonds P et al. (2018).