Apatite is ubiquitous in igneous, metamorphic, and sedimentary rocks and is significant to more fields of study than perhaps any other mineral. To help understand why, one needs to know apatite's structure, composition, and crystal chemistry. Apatite has a robust hexagonal atomic framework based on two distinct metal-cation sites (M1, M2), a tetrahedral-cation site (T), and an anion column along four edges of the unit cell. These cation and anion sites can, among them, incorporate more than half of the long-lived elements in the periodic table, giving rise to the “apatite supergroup,” which contains over 40 mineral species. The structure and composition impart properties that can be technologically, medically, and geologically very useful.