There is extensive antigenic and genomic diversity among co-circulating human rotaviruses. They are differentiated into groups, subgroups and types. There are at least 7 groups (A-G) and 4 subgroups within group A. To distinguish types within group A, a dual classification system has been established with the glycoprotein VP7 defining G types, and the protease-sensitive protein VP4 defining P types. At least 14 G types and more than 20 P types have been distinguished, of which at least 10 G types and at least 11 P types have been found in humans. Using the typing system, the complex molecular epidemiology of rotaviruses was investigated. Rotaviruses of different G and P types co-circulate. The main types found are G1P1A, G2P1B, G3P1A, G4P1A; their relative incidence rates change over time in any one location and are different at the same time between different locations. Viruses with G/P constellations such as G1P1B and G2P1A are mostly natural reassortants of the co-circulating main virus types emerging after double infection of hosts. Viruses carrying G and or P types not represented in the four most common types, e.g. G8P, G1P or G9P, could be introduced into the population by reassortment with animal viruses, or directly from animals or exotic human sources. Naturally circulating rotaviruses constantly undergo point mutations which can be used to classify lineages and sublineages within types. The full significance of human infections with group B and C rotaviruses remains to be established. Surveillance of rotavirus types in different parts of the world is essential to monitor the emergence of new types or of new G/P constellations which may predominate over time. The efficacy and effectiveness of any future rotavirus vaccine may differ depending on the predominant natural strain types. Detailed epidemiological and molecular surveillance data should be utilized to study the transmission dynamics of rotaviruses.