Bainite is a two-phase, or sometimes a three-phase mixture, but its defining feature is the platelet of ferrite that determines the evolution of the remaining components of its structure. To understand other features of the transformation, it is necessary first to consider the atomistic mechanism; the shape deformation caused by bainite growth and the transformation temperatures involved rule out the diffusion of host or substitutional solutes. The plate that forms without any change in composition, is the most difficult to isolate, but there are extraordinary vestiges of this initial event that prove the mechanism. The upper and lower bainite are distinguished by the absence or presence respectively, of carbides within the αb platelets, the determining factor being the competition between carbon partitioning and carbide precipitation. If the former is more rapid than the precipitation reaction, an upper bainitic microstructure is inevitable.