Molecular dynamics simulations of the incorporation of fluoride into hydroxyapatite show that fluoride ions are easily incorporated from solution into the surface of hydroxyapatite, but although the formation of a fluorapatite film prevents the onset of apatite dissolution, this fluoride does not segregate into the bulk material but remains at the surface. These findings significantly increase our understanding of the role of fluoride in stabilizing hydroxyapatite in tooth enamel and suggest that only repeated exposure to fluoride will have a lasting effect on the tooth enamel structure and its resistance toward dissolution and dental decay.