Understanding the health eﬀects of environmental pollutants, especially those due to anthropogenic sources, has become increasingly important in public health research. Toxic contaminants in air, water, and land can aﬀect large populations, and exposures are often involuntary. Recently, the ﬁeld of environmental epidemiology has beneﬁted greatly from our ability to leverage health databases developed for administrative or billing purposes. Examples include vital certiﬁcates, medical records, educational testing data, and registries for cancer, birth defects, and neurological diseases. These databases allow researchers to conduct large-scale population-based studies, often covering large geographical regions. Advances in geographic information systems further enhance our ability to obtain detailed spatial and temporal information, enabling environmental exposures to be ascertained. Results from these population studies have played an important role in setting regulatory standards and protecting public health.