Methods for assessing the risk of spontaneous ignition are reviewed. The most widely used engineering method today, the Frank-Kamenetskii method, is reviewed in detail. Further a new improved experimental method, the so-called crossing-point method, is reviewed. This new method considerably speeds up the testing procedure. The Frank-Kamenetskii theory is best suited for application on materials that do not have excessive moisture content and where the reactions can be sufficiently well described by a single order reaction of the Arrhenius type.