Literature review and modeling studies were performed to assess the role of biogenic VOC emissions in the photochemical ozone problem of urban areas. The assessment effort focussed specifically on recent research results reported by Georgia Institute of Technology (GIT) scientists indicating that biogenic VOC's in Atlanta reduce substantially the effectiveness of VOC controls in reducing ambient ozone. The GIT findings were checked for accuracy and for consistency with previous studies and also for applicability in other urban areas. EPA and SAI modelers replicated the GIT modeling study and verified its results and conclusions. A conflict between the GIT study and an earlier study by others was resolved, further establishing the validity of the GIT results. Factors affecting the strength of the biogenic VOC role in the urban ozone problem were found to be the abundance and reactivity of biogenic emissions, the prevailing wind speeds during ozone episodes, the maximum afternoon mixing height, the size of the urbanized area and perhaps other factors also.