The crystal chemistry of enamel apatite is probably the best understood of all the biological apatites. It is an imperfect apatite, low in calcium and hydroxide ions, but rich in substitutional impurities. A few impurities, like fluoride, are beneficial but most appear to disrupt the lattice structure. In this paper, the composition and basic structural features of enamel apatite are described, with particular emphasis on what we presently know, and do not know, about the role impurities play in establishing the fine details of this structure.