This paper compares the results of four recent engineering-economic studies of the potential for energy technologies to reduce greenhouse gas emissions. The review includes a sector-by-sector assessment of specific technology opportunities and their costs, as estimated by (a) five National Laboratories, (b) the Tellus Institute, (c) the National Academy of Sciences, and (d) the Office of Technology Assessment. These studies document that numerous cost-effective, energy-efficient technologies remain underutilized in each end-use sector of the economy. Supply-side options, on the other hand, are generally found to involve some net costs. Demand- and supply-side options benefit from being pursued concurrently because of various interaction effects. In combination, large carbon reductions are possible at incremental costs that are less than the value of the energy saved. An aggressive national commitment involving some combination of targeted tax incentives, emissions trading, and non-price policies is needed to exploit these carbon reduction opportunities.