Widmanstatten ferrite gets its name from the plate-shaped crystals that are arranged in patterns reminiscent of the macrostructures found in some meteorite specimens. The observed shape deformation and other characteristics of Widmanstatten ferrite indicate a glissile interface between Widmanstatten ferrite and austenite. The chapter describes how the motion of the glissile interface can be stifled by obstacles. This feature, commonly referred to as mechanical stabilisation, is unique to displacive transformations. Widmanstatten ferrite shows all the characteristics of a displacive transformation in which only the carbon is partitioned during growth. Widmanstatten ferrite frequently grows from layers of allotriomorphic ferrite in circumstances where the latter has an orientation with the austenite that is consistent with the displacive mechanism by which Widmanstatten ferrite plates grow. The hydride has a crystallography consistent with the theory of martensite, grows as pairs of accommodating plates, with hydrogen diffusion towards the hydride during the growth process.