The production and characterization of aerosols are essential processes in experimental studies of the toxicity of inhaled particles. Certain methods for producing monodisperse aerosols are especially useful for studying the mechanics of deposition and short-term clearance from the lung and for calibration of sampling instrumentation. Polydisperse aerosols are encountered in practical studies of inhalation toxicity and in medical applications of aerosols. The aerodynamic properties of particles are especially significant in bringing about their deposition in the respiratory tract. They can be measured directly with appropriate instrumentation.