Martensite is a metastable structure that forms during athermal (nonisothermal) conditions. This article reviews the crystallographic theory, morphologies, orientation relationships, habit plane, and transformation temperature of ferrous martensite microstructures. It examines the stages of the tempering process involved in ferrous martensite. The article also describes the formation of the martensite structure in nonferrous systems. It concludes with a discussion on shape memory alloys.