Among epidemiologists, there has been increasing interest in the characteristics of communities that influence health. In the United States, the rural health disparity has been a recent focus of attention and made a priority for improvement. While many standardized definitions of urban and rural exist and are used by social scientists and demographers, they are found in sources unfamiliar to health researchers and have largely not been used in public health studies. This paper briefly reviews some available definitions of urban and rural for American geographic subunits and their respective strengths and weaknesses. For example, some definitions are better suited than others for capturing access to health care services. The authors applied different definitions to breast cancer incidence rates to show how urban/rural rate ratio comparisons would vary by choice of definition and found that dichotomous definitions may fail to capture variability in very rural areas. Further study of the utility of these measures in health studies is warranted.