Fluoride is known to have biological effects on bone cells as well as physicochemical effects on bone crystals. This review concentrates on the latter. Fluoride increases the stability of the apatite lattice and decreases the solubility of the apatite crystals. In bone mineral, this ion has been shown to affect bone crystal structure by increasing crystallinity and reducing specific surface area. These changes in turn lead to changes in the chemistry of bone mineral. Bone mineral deposition is delayed by fluoride. This ion does not diffuse into bone already formed, but is incorporated during mineralization. Subsequently fluoride tends to accumulate in the most highly mineralized bone. Bone treated with fluoride has been shown to be more resistant to acid dissolution than normal bone, which would explain the reduced rate of resorption of fluoridated bone. The distribution of fluoride in bone is not uniform, but its net effect is to increase bone mineral density probably by an increased packing of bone crystals. Finally, there is a debate as to whether fluoride produces a bone of different quality. Whether these changes in the quality of bone will prove to be helpful or harmful remain to be determined.