A fundamental step in risk assessment is establishing what level of chemical contamination is safe. Toxicity reference values (TRVs) represent an exposure dose that people can safely be exposed to without adverse effects. Toxicity reference values are used to derive drinking water and soil quality guidelines, and to calculate hazard quotients (HQs) that determine risk to human health in risk assessments. Although TRVs are regularly used, they are often not completely understood. This presentation will explain how TRVs are derived, where you can find TRVs, and identify differences in TRVs derived by different agencies. Case studies will demonstrate why strong risk assessors should consider the basis of TRVs in their risk conclusions.