This chapter develops the scientific basis for understanding the dynamics of aerosols in clean room environments. Such understanding plays a major role in the monitoring and control of airborne particle contamination, and the detection and elimination of particle sources. These nuclei mode particles are usually generated in very dense clouds, resulting in rapid coagulation during the first few seconds, until the concentration is dilute enough to slow the coagulation. Thus, a very high concentration of nuclei mode particles ends up, after a few minutes, as a much lower number concentration of larger particles. Coarse mode particles generated primarily by mechanical abrasion are: skin flakes, dandruff, hair pieces; cosmetics, powders; sneezes, coughs; clothing pieces, paper pieces; and sand and soil from shoes and floor. Accumulation mode particles may come from three sources: nuclei mode particles, coarse mode particles, and accumulation mode aerosols.