Denitrification is increasingly recognized for its ability to eliminate or reduce nitrate concentrations in groundwater. With this awareness comes a desire to predict the rate and extent of denitrification in aquifers. The limiting factor in making predictive models, however, is our limited knowledge of the physical characteristics of this process. This review synthesizes the published literature on natural aquifer denitrification. A background section discusses denitrification requirements and dissimilatory nitrate reduction to ammonium, which occurs in environments similar to those where denitrification occurs, and gives a historical perspective on denitrification. Other sections discuss denitrification with organic carbon serving as the electron donor (heterotrophic denitrification) and with reduced inorganic compounds serving as the electron donor (autotrophic denitrification). The section on heterotrophic denitrification is structured around two tables that summarize natural aquifer denitrification rates reported by laboratory studies and natural aquifer denitrification rates reported by field studies. The section on autotrophic denitrification discusses denitrification with reduced iron and reduced sulfur. Thus far, most studies only consider a single electron donor or donor type, whether heterotrophic or autotrophic. This review demonstrates, however, that multiple electron donors may be present in a given aquifer. Future research efforts are recommended to determine the factors affecting the availability of electron donors and their denitrification rates. Additional research is also suggested on how dissolved oxygen affects denitrification rates and on the factors influencing the partitioning of nitrate reduction products to nitrous oxide, a potential contributor to the destruction of the ozone layer, and to ammonium.