At a time of deep national concern about both the adequacy of the U.S. energy supply and how much cleaner it can become, the question of how the U.S. tax code influences investment in energy generation is a crucial one. This report offers a comprehensive overview of the energy-related provisions of the U.S. tax code and their estimated impact on tax revenues. More important, this report indicates where the U.S. tax regime as a whole is likely to direct energy investment. The term for such an overall measure is the "effective" tax rate -- that is, the total effect of the tax code on investors trying to decide into which part of the energy industry to put an additional dollar. This paper builds on other work on effective tax rates by including in its analysis production and investment tax credits appearing in the code as well as depletion allowances reserved for the petroleum and gas sectors. It also considers energy-specific tax provisions that most previous analyses have not taken into account. By providing more detailed disaggregated estimates than its predecessors, it is able to permit clearer and broader comparisons of the tax code's effects on investment in different fuels. The present analysis has determined that a major shift has occurred since the time, not so long ago, when the tax code encouraged domestic oil and coal investment above all other kinds. The subsidy for fossil fuels has dropped from over 60 percent in 1997 to under 50 percent in 2007. The subsidy for renewable energy and conservation has risen from just under 40 percent to over 50 percent in the same period. The current tax code, especially since enactment of the Energy Policy Act of 2005, strongly encourages investment in nuclear, wind, and solar power, which enjoy tax subsidies ranging from nearly 100 percent, for nuclear, to more than 200 percent, for solar. In other words, tax subsidies for these forms of energy generation are sufficiently generous that investors may use them to offset tax liabilities for capital gains and income derived from non-energy investments. It is worth noting that wind capacity, a highly tax-favored source of energy, grew by nearly 50 percent in 2007 and accounted for one-third of all new electrical capacity added in that year. Independent oil companies that are able to use percentage depletion to the fullest extent have also received significant tax benefits at the margin. Still, the positive impact of these tax subsidies is to some extent vitiated by the code's relatively ungenerous treatment of investment in the electric grid, which carries electricity produced by any type of energy source to businesses and households. Given the great distance of the steadiest sources of wind and solar power from the largest energy consumers, the economics of these cleaner sources depend on the further development of high-voltage transmission lines and other features of the grid. Yet the code continues to tax income realized from investments in high-voltage power transmission lines more heavily than capital gains or most ordinary income.