We are using mathematical models to understand patterns of incidence and potential "herd immunity" effects as rotavirus vaccination begins to be rolled out in developing countries.In 2009, WHO recommended the global use of rotavirus vaccines, and with the financial support of GAVI, many developing countries will soon begin vaccination against rotavirus. Modeling the dynamics of rotavirus is essential for understanding the dynamical consequences and potential indirect, or “herd immunity” effects of vaccination. Our earlier work has shown that the changes in the timing of rotavirus epidemics, age distribution of cases, and distribution of genotypes causing infection can be explained by understanding the underlying transmission dynamics. However, the impact of vaccination may differ in developing countries, where immunity from natural infection is weaker, vaccine efficacy is considerably reduced, and seasonal patterns of incidence and genotypes causing infection differ. We are in the process of extending our model for the transmission dynamics of rotavirus to explain the differences in pre-vaccination patterns of epidemics in developing countries, including Bangladesh and Malawi. We will then examine the potential impact of moderately efficacious vaccines on the incidence, age distribution, and seasonal pattern of epidemics in these settings.