A majority of the world's population will live in urban areas by 2007. The most rapidly urbanizing cities are in less-wealthy nations, and the pace of growth varies among regions. There are few data linking features of cities to the health of populations. We suggest a framework to guide inquiry into features of the urban environment that affect health and well-being. We consider two key dimensions: urbanization and urbanicity. Urbanization refers to change in size, density, and heterogeneity of cities. Urbanicity refers to the impact of living in urban areas at a given time. A review of the published literature suggests that most of the important factors that affect health can be considered within three broad themes: the social environment, the physical environment, and access to health and social services. The development of urban health as a discipline will need to draw on the strengths of diverse academic areas of study (e.g., ecology, epidemiology, sociology). Cross-national research may provide insights about the key features of cities and how urbanization influences population health.