The successful development and implementation of rotavirus vaccines will have a significant impact on rotavirus- induced gastroenteritis in children worldwide. However, this introduction will increase the immune pressure against wild-type rotavirus strains circulating in the community, altering the forces and balances that drive rotavirus evolution. There is concern that antigenically distinct novel or rare strains may be selected and spread, decreasing vaccine efficacy. This review describes the diversity of rotavirus strains and the mechanism by which wild-type rotavirus strains can evolve, including genetic drift and genetic reassortment.