Rotaviruses cause extensive morbidity and mortality worldwide, thus corroborating the need for a vaccine that is effective in all socioeconomic environments. Vaccines evaluated in clinical trials have all been live attenuated rotaviruses that are delivered orally to mimic the excellent protection observed after natural infection. The mechanisms by which these vaccine candidates or wild-type rotaviruses elicit protection are not fully understood. During the 1980s, several candidate vaccines provided little protection, particularly in developing countries, and were discontinued. Two, however, are in the process of being licensed worldwide, and several others are undergoing clinical trials. Development of live rotavirus vaccines has been highly influenced by views regarding the importance of serotype-specific neutralizing antibody. Development of several candidate vaccines is based on the concept that neutralizing antibody is the primary determinant of protection. These candidates, including 1 of the 2 being licensed worldwide (RotaTeq), are composed of multiple rotavirus strains representative of the major human rotavirus serotypes. The other group of candidates has been developed based on the theory that protection is not solely dependent on neutralizing antibody. These candidates are composed of single rotavirus strains and include the other vaccine being licensed worldwide (Rotarix). Studies that provide the basis for each approach will be presented and discussed.