Design theory for cascade impactors is well developed, and design principles can be summarized quite succinctly. The key geometric parameters of three commonly used impactors have been compared to design guidelines. Calibration theory and practice have undergone improvement in recent years. Published results show distinctly different characteristics for three commonly used impactors, and conformance to modern design recommendations results in desirable performance characteristics. A consideration of the sensitivity of stage collection efficiency characteristics to geometric variables and flow rate indicates that measurement of certain physical dimensions is sufficient to assure that impactor performance matches that of a properly calibrated unit. Flow rate is an important operating variable that is more likely to fall out of its calibrated range than important dimensional variables resulting from instrument manufacturing. In practice, data from impactors are frequently treated as though the impactors have ideal collection characteristics. The practical effects of impactor nonideal performance can be demonstrated by model calculations, and these show the necessity of data inversion to obtain a size distribution. The process is not straightforward for unknown distributions. There are much smaller differences between ideal and real performance for an impactor complying with design guidelines, thus, the cutoff characteristics are sharp.