EPA is issuing a policy and preliminary guidance on using probabilistic analysis. The policy documents the EPA's position that such probabilistic analysis techniques as Monte Carlo analysis, given adequate supporting data and credible assumptions, can be viable statistical tools for analyzing variability and uncertainty in risk assessments. The policy also establishes conditions that are to be satisfied by risk assessments that use probabilistic techniques. These conditions are in keeping with the Agency's risk characterization policy that requires clarity, consistency, transparency, and reproducibility in risk assessments. The report presents a general framework and broad set of principles important for ensuring good scientific practices. Many of the principles apply generally to the various techniques for conducting quantitative analyses of variability and uncertainty; however, the focus of the principles is on Monte Carlo analysis. The guiding principles are intended to serve as a minimum set of principles and are not intended to constrain or prevent the use of new or innovative improvements where scientifically defensible.