Measurement of suspended particulate matter in the air is likely the oldest form of air quality sampling. High-volume samplers, PM10 samplers, dichotomous samplers, polyurethane foam samplers, industrial hygiene personal samplers, and asbestos samplers all employ filtration sampling and require flow monitoring and sampler air volume measurement. The basic applications of filtration theory to particulate air sampling are reviewed in this chapter. Sections 9.7 and 9.8 give specific details on calibration techniques.