Crystals characterized by oscillatory zoning show evidence for a quasi-cyclic alternation in the chemical composition of discrete growth-shells from tens of nanometers to tens of micrometers in thickness. Oscillatory zoning is most commonly observed and studied in crystals of magmatic plagioclase, but sensitive imaging techniques (e.g., Nomarski interference contrast microscopy, cathodoluminescence, back-scattered electron imaging, X-ray topographs) reveal its presence in many other minerals. Through the application of these techniques and an exhaustive review of the literature, oscillatory zoning is shown to occur in at least 75 rock-forming and accessory minerals comprising most major mineral classes: silicates (hydrous and anhydrous), sulfides, oxides, halides, carbonates, phosphates, and sulfates. Such mineral zoning is a common but often well-concealed phenomenon in magmatic rocks (particularly alkaline ones), hydrothermally altered rocks, mineralized rocks, and carbonate sequences. Mechanisms of oscillatory zoning are discussed, including the less-studied effects of adsorption and elastic stress in near-surface growth-layers. Its "non-equilibrium chemistry" makes oscillatory zoning a rich subject for further research.