Rotaviruses (RVs) are the main cause of acute gastroenteritis in infants and young children worldwide and in the young of many animals. They are classified into at least five groups (A to E), and there are possibly two more groups (F and G) according to epitopes on the viral protein 6 (VP6) protein. Within group A, four subgroups (SGs) have been differentiated according to the epitopes on the VP6 middle layer protein. The two outer layer proteins, VP7 and VP4, form the basis of the present dual classification system of group A RVs into G and P types. At least 14 different G-serotypes, G1 to G14, have been identified among human and animal RVs, based on differences in the neutralization epitopes of the VP7 protein. Of these, at least 10 are associated with infection in humans. Twenty different P types have been differentiated by sequence analysis of their VP4 genes, of which at least 10 have been found in human isolates. This chapter describes the variability of group A RVs with regard to SGs and G and P types. It reviews the mechanisms of genomic change underlying the evolution of these viruses.