Cofiring biomass with coal in existing power plants offers a relatively inexpensive and efficient option for increasing near-term biomass energy utilization. Potential benefits include reduced emissions of carbon dioxide, sulfur, and nitrogen oxides and development of biomass energy markets. To understand the economics of this strategy, we develop a model to calculate electricity and pollutant mitigation costs with explicit characterization of uncertainty in fuel and technology costs and variability in fuel properties. The model is first used to evaluate the plant-level economics of cofiring as a function of biomass cost. It is then integrated with state-specific coal consumption and biomass supply estimates to develop national supply curves for cofire electricity and carbon mitigation. A delivered cost of biomass below $15 per ton is required for cofire to be competitive with existing coal-based generation. Except at low biomass prices (less than $15 per ton), cofiring is unlikely to be competitive for NO x or SO x control, but it can provide comparatively inexpensive control of CO 2 emissions: we estimate that emissions reductions of 100 Mt-CO 2 /year (a 5% reduction in electric-sector emissions) can be achieved at 25 ( 20 $/tC. The 2-3 year time horizon for deploymentscompared with 10-20 years for other CO 2 mitigation optionssmakes cofiring particularly attractive.