The microstructural basis of the critical tensile stress criterion of cleavage fracture is discussed. It is shown that cleavage can be carbide induced in bainitic steels as well as in ferritic steels. The plastic work expended in brittle crack propagation is shown to be somewhat smaller for bainitic steels than for ferritic steels. This is proposed to be caused by the much higher dislocation density of the bainitic microstructure. The statistical nature of the cleavage fracture initiation process is shown to result in a test piece geometry dependent cleavage fracture stress.