Substantial knowledge has been generated in the U.S. about the resource base for forest-and other residue-derived biomass for bioenergy including co-firing in power plants. However, a lack of understanding regarding power plant-level operations and manager perceptions of drivers of biomass co-firing remains. This study gathered information from U.S. power plant managers to identify drivers behind co-firing, determine key conditions influencing past and current use, and explore future prospects for biomass in co-firing. Most of the biomass used in co-firing was woody biomass procured within 100 km of a power plant. Results show that the most influential co-firing drivers included: adequate biomass supply, competitive cost of biomass compared to fossil fuels, and costs of biomass transport. Environmental regulations were generally considered second-most influential in decisions to test or co-fire with biomass, but were of high importance to managers of plants that are currently not co-firing but may in the future. (C) 2013 Elsevier Ltd. All rights reserved.