Extreme heat events are likely to become more frequent in the coming decades due to climate change. Exposure to extreme heat depends not only on changing climate, but also on changes in the size and spatial distribution of the human population. A recent analysis provides a new projection of population exposure to extreme heat for the continental United States that takes both of these factors into account. Using projections from a suite of regional climate models driven by global climate models and forced with the A2 scenario from the Special Report on Emissions Scenarios by the Intergovernmental Panel on Climate Change and a spatially explicit population projection consistent with the socioeconomic assumptions of that scenario, changes in exposure are projected into the latter half of the 21st century. The results show that U.S. population exposure to extreme heat increases four- to six-fold over observed levels in the late 20th century, and that changes in population are as important as changes in climate in driving this outcome. Aggregate population growth, as well as redistribution of the population across larger U.S. regions, strongly affects outcomes while smaller-scale spatial patterns of population change have smaller effects. The relative importance of population and climate as drivers of exposure varies across regions of the country. This research was funded in part by the Office of Biological and Environmental Research’s Integrated Assessment Research and Regional and Global Climate Modeling programs.