Although burning fossil fuels has environmental consequences, many countries have switched away from nuclear power in favor of fossil-fuel fired electricity production after incidents at Three Mile Island, Chernobyl, and Fukushima. This study estimates the substitution between nuclear and fossil-fuel fired electricity generation in the United States. Using an event-study framework, we leverage nuclear plant openings from 1970 to 1995 and forced nuclear plant outages from 1999 to 2014. Plant openings (nuclear outages) reduce (increase) monthly net coal-fired generation by approximately 200 GWh, implying a considerable reduction (increase) in emissions. We find that the substitution between nuclear and coal is not one-to-one, as has been assumed in prior literature. After establishing these stylized facts, we explore the potential underlying forces driving the observed substitution between coal and nuclear.