In past studies evaluating whether health problems experienced by Vietnam veterans might be linked to wartime use of Agent Orange or other herbicides, a fundamental challenge has been a lack of information about the veterans' level of exposure to these herbicides. To address that problem, researchers developed a model to assess the opportunity for herbicide exposure among these veterans. Using a geographic information system and a computerized database engine, the model makes it possible to link georeferenced data on herbicide spray paths with information about troop locations and calculate two different proximity-based exposure opportunity metrics. This report presents... guidance to the Dept. of Veterans Affairs about the best use of this herbicide exposure assessment model.