In 1994, the Forest Inventory and Analysis (FIA) and Forest Health Monitoring programs of the U.S. Forest Service implemented a national ozone (O3) biomonitoring program designed to address specific questions about the area and percent of forest land subject to levels of O3 pollution that may negatively affect the forest ecosystem. This is the first and only nationally consistent effort to monitor O3 stress on the forests of the United States. This report provides background information on O3 and its effects on trees and ecosystems, and describes the rationale behind using sensitive bioindicator plants to detect O3 stress and assess the risk of probable O3 impact. Also included are a description of field methods, analytic techniques, estimation procedures, and how to access, use and interpret the ozone bioindicator attributes and data outputs such as the national ozone risk map.