The same fuels are taxed at widely different rates in different countries while different fuels are taxed at widely different rates within and across countries. This paper considers what tax theory has to say about efficient energy tax design. The main factors for energy taxes are the optimal tariff argument, the need to correct externalities such as global warming, and second-best considerations for taxing transport fuels as road charges, but these arc, inadequate to explain current energy taxes. EU energy tax harmonisation and Kyoto suggest that the time is ripe to reform energy taxation.