Electric vehicles (EVs) are often promoted as a solution to urban air pollution, but their environmental impact depends heavily on how the electricity that powers them is generated. In regions where the electrical grid relies primarily on coal, driving an EV can produce nearly as many greenhouse gas emissions as driving an efficient gasoline car. However, in regions powered by renewable energy or natural gas, EVs produce significantly fewer emissions. As the grid becomes cleaner over time, the environmental advantage of EVs is expected to grow.