Purpose: This Engineer Manual (EM) provides U.S. Army Corps of Engineers (USACE) and other personnel with procedural guidance to develop Conceptual Site Models (CSMs) at sites potentially containing ordnance and explosives (OE) and/or hazardous, toxic, and radioactive waste (HTRW) environmental contamination. The CSM is a description of a site and its environment that is based on existing knowledge. It describes sources and receptors, and the interactions that link these. It assists the team in planning, interpreting data, and communicating. The CSM will provide a planning tool to integrate information from a variety of resources, to evaluate the information with respect to project objectives and data needs, and to respond through an iterative process for further data collection or action. The target audience is the project delivery team (PDT).