A conceptual site model (CSM) is a representation of the relationships among key site features that pertain to the decisions that need to be made. CSMs can take many forms, from a written description of contaminant release and transport mechanisms, to simple schematics, to complicated 3-dimensional computer models of contaminant state, fate, and transport. The form and complexity of a CSM is determined by the significance of the decision to made (i.e., the implications of being wrong) and the level of site understanding required to make that decision. Using CSMs makes projects more cost-effective because a CSM delineates the contaminant populations for which exposure and remediation decisions are different, avoiding inappropriate classification of on-site media. A CSM is by nature evolutionary, changing and becoming more accurate as additional information is gathered pertinent to the site and its conditions. The Triad approach provides the mechanism for performing this updating process while data collection is underway. CSMs are critical components of the Triad systematic planning process. They capture all key site information pertinent to the decisions that need to be made, and allow an explicit evaluation of the uncertainty associated with those decisions. If that uncertainty is considered unacceptable, the CSM assists in developing uncertainty management approaches, including identifying key data gaps that, if filled, would allow decision-making to proceed at acceptable levels of confidence.