This report updates earlier work by Metcalf and Weisbach (2009) on design considerations for a national carbon tax. It maintains that 75 to 85 percent of US greenhouse gas emissions could reasonably be covered by a carbon tax. In contrast to the earlier paper, it argues that natural gas should be taxed downstream, given the large fraction of marketed gas that does not go through processing plants. The report also describes various approaches to setting the tax rate on emissions and suggests that a Pigouvian approach where the tax rate is periodically updated to reflect new estimates of the social cost of carbon (and other greenhouse gases) reasonably approximates the optimal nonlinear carbon tax. Finally, it discusses the interplay between federal and state carbon pricing policies.