The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency (EPA) to assess the cumulative risks to human health that can result from exposure to pesticides and other substances that are toxic by a common mechanism. The Agency is currently developing a process for performing cumulative risk assessments of this type. Such assessments will play an increasingly important role in the evaluation of risks posed by pesticides, and will improve the Agency's ability to make regulatory decisions that fully protect public health and sensitive subpopulations, including infants and children. The identification of pesticides and other substances that cause a common toxic effect by a common mechanism is the first step of the cumulative risk assessment process. This document describes the approach that EPA will use for identifying pesticides and other substances that cause common toxic effects by common mechanisms of toxicity.