AbstractEnvironmental assessment consists of scientific studies to define contamination at a potential release site or sites and to evaluate the risk posed to human health and the environment. These studies are performed within a prescribed regulatory framework. There is a high degree of uncertainty associated with preparing cost and schedule estimates for activities such as site characterization, risk assessment, and evaluation of remediation alternatives. This article describes the approach that Sandia National Laboratories is using to meet the challenge of estimating the assessment phase of its Environmental Restoration Project. Emphasis is placed on lessons learned, with examples given to illustrate the approach.