The aim of this review has been to suggest that future fluoride preventive programs should be based on what is at present known about the possible cariostatic mechanisms of fluoride. The relative role of enamel fluoride in caries prevention has become increasingly questioned during the last two decades. No significant relationship has been demonstrated between caries experience of the individual and fluoride content of the enamel. Furthermore, the fluoride content in surface enamel between teeth developed in low and "optimal" fluoride areas is too small to explain any significant effect on dissolution rate of the enamel. As the major explanation for the cariostatic effect of fluoride must therefore be sought in its local effect on the oral environment, the possible effects on plaque colonization, composition and metabolic activities are discussed. The effect of even low concentrations of fluoride in the liquid phase on enamel dissolution is described, and it is concluded that the major cariostatic effect of water fluoridation, fluoride tooth paste and mouth rinses can probably be ascribed to regular increases in fluoride ion activity in the oral fluids. The effect of high concentrations of topical fluoride solutions is thought to be a result of a slow dissolution of calcium fluoride deposited in initial caries lesions, whereby an increased fluoride ion concentration is maintained locally for longer periods of time. Finally, the clinical consequences of these considerations are described.