Policies that reduce emissions of greenhouse gases can simultaneously alter emissions of conventional pollutants that have deleterious effects on human health and the environment. This paper first describes how these "ancillary" benefits--benefits in addition to reduced risks of climate change--can result from greenhouse gas (GHG) mitigation efforts. It then discusses methodologies for assessing ancillary benefits and provides a critical review of estimates associated with reductions of criteria air pollutants. We find that these benefits in the U.S. may be significant, indicating a higher level of "no regrets" greenhouse gas abatement than might be expected based on simple economic calculations of abatement cost. However, the magnitude of ancillary benefits realized by any program of GHG mitigation is highly dependent on the location, pollutant, degree of exposure, and the economic behavior of individuals in response to the program. It is also highly dependent on the interaction of GHG abatement policies with the policies used for regulating conventional pollutants. We identify a rule of thumb to suggest ancillary benefits could be on the order of 30 percent of the incremental cost of GHG mitigation. For modest carbon reduction that do not result in changes in emissions of sulfur dioxide by electric utilities, ancillary benefits may be as high as $7 per ton. Greater benefits could be obtained with larger GHG reductions, although the costs of abatement would also be much greater.