Plaque, a causal factor for caries and periodontal diseases, is a habitat for various microorganisms. The considerable qualitative and quantitative variation of microorganisms, predominantly Streptococcus mutans, depends on age, thickness, and acidogenicity. The food passing through the mouth is also a crucial factor. Plaque removal (plaque control) is the most important measure for prevention of caries and gingivitis. Since mechanical removal of plaque is not adequate, chemical agents must also be used. Fluoride, for example, effectively inhibits plaque metabolism. The efficacy of enzymes, alkaloids, and antiseptics is limited. No active immunization against cariogenic streptococci is available at present. Mechanical, chemical, and biological measures of plaque control are effective only when the population is motivated to practice oral hygiene.