There is widespread agreement among economists — and a diverse set of other policy analysts — that at least in the long run, an economy-wide carbon pricing system will be an essential element of any national policy that can achieve meaningful reductions of CO2 emissions cost-effectively in the United States. There is less agreement, however, among economists and others in the policy community regarding the choice of specific carbon-pricing policy instrument, with some supporting carbon taxes and others favoring cap-and-trade mechanisms. This prompts two important questions. Which — if either — of the two major approaches to carbon pricing is superior in terms of relevant criteria, including but not limited to efficiency, cost-effectiveness, and distributional equity? And which of the two approaches is more likely to be adopted in the future in the United States?