The term "rural" suggests many things to many people, such as agricultural landscapes, isolation, small towns, and low population density.However, defining "rural" for health policy and research purposes requires researchers and policy analysts to specify which aspects of rurality are most relevant to the topic at hand and then select an appropriate definition. Rural and urban taxonomies often do not discuss important demographic, cultural, and economic differences across rural places-differences that have major implications for policy and research. Factors such as geographic scale and region also must be considered. Several useful rural taxonomies are discussed and compared in this article. Careful attention to the definition of "rural" is required for effectively targeting policy and research aimed at improving the health of rural Americans.