Abstract The environmental costs of energy supply have been rising, reinforcing the effect of increased monetary costs in creating incentives for increasing the efficiency with which energy is used. Quantifying these environmental costs is difficult, but it is instructive to try. The estimates presented here for the direct public-health damages of electricity generation with coal and nuclear power show overlapping uncertainties, with no clear basis for preferring either of these energy sources over the other on these grounds. Impacts of energy supply on climate and ecosystems—such as through carbon dioxide accumulation in the atmosphere and acid precipitation—may ultimately do even greater damage to human well-being than the more publicized and more readily quantified air-pollution and accident hazards. Systematic comparison of energy supply with other industrial activities and with agriculture as a cause of regional and global environmental disruptions confirms the widespread impression of energy's key role in large-scale environmental problems.