Steels, when they transform completely into allotriomorphic ferrite, have a microstructure of space-filling equiaxed ferrite grains, the shape of which bears little resemblance to the early stages of transformation. Massive ferrite, which also grows by a reconstructive transformation mechanism, has the distinction that it inherits the composition of the parent austenite. All of the transformations associated with the decomposition of austenite are thermodynamically of first order. They occur by the propagation of well-defined interfaces that can be coherent, semi-coherent or incoherent. Coherency can of course be forced when the particle is small with consequent strain energy; stress-free coherence refers to the case where there is no breakdown of coherency as the particle grows. The crystallography of grain boundary nucleated ferrite indicates that the ferrite has a good-fit orientation relationship with at least one of the adjacent austenite grains. It is observed routinely that the orientation relationships that develop during phase transformations in the solid-state are not random.