A September 2008 workshop sponsored by the Society for Risk Analysis(1) on risk assessment methods for nanoscale materials explored “nanotoxicology” in risk assessment. A general conclusion of the workshop was that, while research indicates that some nanoscale materials are toxic, the information presented at the workshop does not indicate the need for a conceptually different approach for risk assessment on nanoscale materials, compared to other materials. However, the toxicology discussions did identify areas of uncertainty that present a challenge for the assessment of nanoscale materials. These areas include novel metrics, characterizing multivariate dynamic mixtures, identification of toxicologically relevant properties and “impurities” for nanoscale characteristics, and characterizing persistence, toxicokinetics, and weight of evidence in consideration of the dynamic nature of the mixtures. The discussion also considered “nanomaterial uncertainty factors” for health risk values like the Environmental Protection Agency's reference dose (RfD). Similar to the general opinions for risk assessment, participants expressed that completing a data set regarding toxicity, or extrapolation between species, sensitive individuals, or durations of exposure, were not qualitatively different considerations for nanoscale materials in comparison to all chemicals, and therefore, a “nanomaterial uncertainty factor” for all nanomaterials does not seem appropriate. However, the quantitative challenges may require new methods and approaches to integrate the information and the uncertainty.