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 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 greenhouse gas 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 greenhouse gas abatement policies with the policies used for regulating conventional pollutants.