Accurate and precise flow rate and gas concentration measurement standards are needed for comparable air quality measurements. Transfer standards are most often used for calibration, performance testing, and auditing of field monitors. These must be traceable to primary standards that are in turn derived from fundamental units of length, mass, temperature, and time. Flow rates and volumes are measured by devices based on positive displacement, pressure differences, and temperature increases or decreases. Stable gas concentrations are prepared in non-reactive pressurized containers by gravimetric and dilution methods. Reactive gases are generated from photochemistry and permeation devices. These standards are used to determine the accuracy, precision, and validity of air quality measurements, and these attributes should be reported with the measurement values.