Within the field of cosmetic science, cosmetic microbiology is a dynamic discipline which is growing to meet the needs of scientists in the cosmetic and pharmaceutical industries. Cosmetic microbiology was, in its first 50 years, primarily concerned with the microbiological asepcts of plant sanitation, determining aerobic plate counts of finished products, and preservative efficacy testing. Changes are being made in the cosmetic products and claims. Companies are introducing products that are more therapeutically positioned, that claim increased performance in treataing specific conditions, and that meet changed consumer needs. These developments are cause a shift of focus in cosmetic microbiology from formula development and manufacturing plant sanitation concerns to clinical studies on the effects of active ingredients in drug products and microorganisms on the skin. This monograph discusses issues relevant to cosmetic and drug microbiology.