Group A rotaviruses have been recognized worldwide as the main viral cause of acute infantile gastroenteritis and of diarrhoea of the young of a wide variety of animal species. The molecular epidemiology of rotaviruses has revealed an extensive genomic and antigenic variability of successively occurring or cocirculating human group A rotaviruses. Group B rotaviruses have been found to cause outbreaks of acute gastroenteritis among adults in China. The genomic and antigenic diversity of human rotaviruses have evolved by means of the following mechanisms: 1. sequential point mutations of the genomic RNA; 2. formation of reassortants; 3. introduction of an animal rotavirus into the human population; 4. genome rearrangements; 5. combinations of factors 1-4. The analysis of the functions of certain gene products forms the basis for a molecular pathology of rotaviruses.