Abstract
In 2006, the U.S. celebrates the 100th birthday of the census tract. These geographic units, born out of concerns for urban well-being, were first proposed in 1906 to provide a "convenient and scientific city map system" for the City of New York. They were employed for the first time in the U.S. census in 1910 in eight cities, via a joint effort involving the U.S. Census Bureau and state and local health departments. Initially termed "sanitary areas" because of their relevance to planning for public health and health services, census tracts are now widely used by all sectors of government and by myriad disciplines in the health, social, and geographic sciences for research as well as policy development, implementation, and evaluation. In this article, I describe the census tract's underappreciated origins, give examples of its current use in analyzing and addressing social disparities in health and health care, and discuss its continued significance and implications for population health and the public data required for informed democratic governance.