Harnessing renewable energy for the production of fuels holds the potential to curb our dependence on fossil fuels, reduce greenhouse gas emissions, and stimulate the global economy. Technologies such as electrofuels, artificial photosynthesis, and biofuels are emerging that promise renewable fuels at competitive costs and at any scale. What are the distinguishing and enabling elements of these technologies? What new opportunities do renewable fuels afford? What are the metrics by which the next fuel will be judged?