AbstractSystemic fluoride may enhance the resistance of the tooth by way of (1) an alteration in tooth morphology, and (2) a conversion of the hydroxyapatitic mineral to a fluoridated state with an attendant reduction in solubility and an enhancement of the re mineralization phase of the caries process. It has been reported by a number of investigators, although it is not universally accepted, that the posterior teeth from fluoridated areas have a distinct morphology that is less susceptible to caries attack. Fluorapatite and hydroxy apatite do not differ appreciably with respect to the amount of mineral that dissolves at a specified concentration of acid. However, if solubility is considered as a function of pH at equilibrium, i.e., the final pH after the dissolution products have entered the solvent‐a model more akin to the in vivo situation‐hydroxyapatite is the conspicuously more soluble of the two minerals. In this connection, epidemiologic data suggest that children with unusually high levels of surface enamel fluoride have little or no caries. This observation is consistent with the finding that the solubility of solid apatite solutions with varying degrees of fluoride substitution was inversely related to fluoride content, with minimal solubility at 0.5 substitution, a value observed in natural teeth at the very outermost surface of the enamel. Evidence of a preemptive effect of fluoride has also been shown in field studies in which subjects had reduced levels of caries with fluoride exposure in childhood o