Micaceous iron oxide (MIO) is a crystalline form of iron oxide that differs from the more familiar red, yellow, and brown forms of iron oxide pigments. Its crystal are very easily fractured into thin flakes, giving it a physical form similar to mica. The use of coatings containing MIO pigments is increasing rapidly, accelerated by globalization of the manufacturing and specifying industries. How these pigments work, the factors that affect performance, and key specifying criteria are discussed.