The relatively high rate of genetic drift observed with pestiviruses results not only in the emergence of new species but also in heterogeneity within recognized species. The grouping of pestiviruses into species was first based on host and to a lesser extent on clinical presentation, then later by antigenic comparisons and more recently genetic comparisons. Subgrouping within species is a more recent taxonomic development and has been based primarily on genetic comparisons. While there are a number of publications proposing new species within the Pestivirus genus or new subgroups within recognized pestivirus species, there is no consensus on the criteria required to declare a new species and subgroup. Defining the criteria used for segregation of species and subgroups within a species is important because differentiating viral species and subspecies and determining the regions in which they are endemic has profound implications for eradication efforts, the design of vaccines and for regulation of trade in livestock and animal derived products, such as fetal bovine serum. This review outlines criteria used for differentiating species within the different genera of the Flaviviridae, compares criteria previously used for differentiating pestiviruses and discusses the need for the development of a unified system for defining species and subgroups within the Pestivirus genus.