Some analysts think that biomass offers great potential as a renewable fiel for electricity generation. Biomass’ emissions of greenhouse gases, over its whole fiel cycle, are relatively insignificant when compared to even the “cleanest” of the fossil energy technologies — natural gas. Yet, the external costs (and benefits) of biomass fuel cycles, which stem from impacts other than those associated with greenhouse gases, are not necessarily trivial. Some of these impacts are similar to those that result from the combustion of fossil fbels. Other impacts are unique to biomass fiel cycles. This report describes methods to estimate many of the more important external costs (and benefits) of biomass fiel cycles.