The effect of biodiversity on natural communities has recently emerged as a topic of considerable ecological interest. We review studies that explicitly test whether the number of species in a community (species richness) regulates the temporal variability of aggregate community (total biomass, productivity, nutrient cycling) and population (density, biomass) properties. Theoretical studies predict that community variability should decline with increasing species richness, while population variability should increase. Many, but not all, empirical studies support these expectations. However, a closer look reveals that several empirical studies have either imperfect experimental designs or biased methods of calculating variability. Furthermore, most theoretical studies rely on highly unrealistic assumptions. We conclude that evidence to support the claim that biodiversity regulates temporal variability is accumulating, but not unequivocal. More research, in a broader array of ecosystem types and with careful attention to methodological considerations, is needed before we can make definitive statements regarding richness-variability relationships.