The paper treats micro-mechanical modes of crack extension, classed as 'cracking' and 'rupture' processes. In 'cracking', a cleavage crack nucleus propagates when a critical local tensile stress is attained, the magnitude of the stress being determined by the microstructure. Models for crack propagation from carbides and from martensite/bainite 'packets' are discussed. The 'rupture' processes involve the initiation and growth of voids, centred on second-phase particles. Coalescence may arise from 'internal necking' or 'fast shear' and the factors associated with these two modes are described. Consideration is also given to the ways in which microstructure may produce scatter in toughness values and in growth-rates under fatigue loading, where both cyclic and monotonic failure modes are significant.