The U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Stationary Source Compliance Division, as well as the Emission Monitoring Science Laboratory, Quality Assurance Division, and several regional offices have sponsored continuous emission monitoring system (CEMS) performance audits to determine the precision and accuracy of CEMS and to develop and evaluate the audit techniques. Opacity, SO2 and NOx CEMS at a variety of industrial sources (electric utilities, manufacturing facilities, petrochemical processes, kraft pulp mills, etc.) were audited using NBS traceable filters, EPA reference methods, and calibration gases are reference standards. This report provides a summary of the results of these audits. The results demonstrate: 80% of the opacity monitors audited met calibration error criteria (5% error) for low- and mid-range references values*; 65% of the monitors met these criteria for high range reference values, 80% of the gas CEMS met the calibration error criteria (5% error), and 80% of the gas CEMS met the relative accuracy criteria (25% relative accuracy).