At first glance, the stability of polar faces of ionic crystals seems a mystery. Because their layers are alternately made up of oppositely charged ions, a net dipole moment accumulates along the crystal’s c axis so in principle, the electrostatic energy of a macroscopic crystal bounded by polar surfaces should diverge proportionally as the sample thickness. Nonetheless, a number of stable ionic polar faces occur in nature. Among these, the Zn-terminated (0001) and O-terminated (000-1) faces of ZnO have received widespread attention because these surfaces are active in catalytic and corrosion reactions. In this talk, I shall explain the competing forces acting to stabilize the polar surfaces of ZnO.