Using multidisciplinary data and methods from the social, biophysical, and health sciences, this project investigates the vulnerability of different neighborhoods and groups of people to negative health effects of extremely hot weather in metropolitan Phoenix. There are two overarching hypotheses in this project: 1) Urbanization in the region has produced (and continues to reproduce) segregated residential neighborhoods with increasing levels of environmental and social inequalities. 2) This heterogeneity in living conditions has rendered low-income and racial/ethnic minority populations disproportionately vulnerable to heat-related health hazards that are characteristic of the summer climate in Phoenix. This project builds on and expands research that shows substantial spatio-temporal variation in Phoenix’s near-surface urban heat island (UHI). We correlate variation in physical characteristics of the UHI with socioeconomic and land cover characteristics of neighborhoods in order to determine differences in exposure to high temperatures, sensitivities of residents to heat-related illnesses, and coping capacities of households to deal with hot weather. The health outcomes evaluated in relation to the spatial form, intensity, and dynamics of the UHI are morbidity and mortality from heat-related causes in Maricopa County during the last decade.