Cleavage fracture in ferritic steels can be defined as a sequence of few critical steps. At first nucleation of a microcrack takes place, often in a hard inclusion. This microcrack then propagates into the surrounding matrix material. The last obstacle before failure is the encounter of grain boundaries. If a microcrack is not arrested during any of those steps, cleavage takes place. Temperature plays an important role since it changes the failure mode from ductile to brittle in a narrow temperature interval. In papers A and B micromechanical models of the last critical phase are developed (cleavage over a grain boundary) in order to examine the mechanics of this phase. An extensive parameter study is performed in Paper A, where cleavage planes of two grains are allowed to tilt relative each other. It is there shown that triaxiality has a significant effect on the largest grain size that can arrest a rapidly propagating microcrack. This effect is explained by the development of the plastic zone prior to crack growth. The effect of temperature, addressed through a change in the visco-plastic response of the ferrite, shows that the critical grain size increases with the temperature. This implies that with an increasing temperature more cracks can be arrested, that is to say that less can become critical and thus that the resistance to fracture increases. Paper B shows simulations of microcrack propagation when the cleavage planes of two neighboring grains are tilted and twisted relatively each other. It is shown that when a microcrack enters a new grain, it first does it along primary cleavage planes. During further growth the crack front is protruded along the primary planes and lags behind along the secondary ones. The effect of tilt and twist on the critical grain size is decoupled with twist misorientation offering a greater resistance to propagation. Simulations of cracking of a particle and microcrack growth across an inclusion-matrix interface are made in Paper C. It is shown that the particle stress can be expressed by an Eshelby type expression modified for plasticity. The analysis of dynamic growth, results in a modified Griffith expression. Both findings are implemented into a micromechanics-based probabilistic model for cleavage that is of a weakest link type and incorporates all critical phases of cleavage: crack nucleation, propagation over particle-matrix interface and into consecutive grains. The proposed model depends on six parameters, which are obtained for three temperatures in Paper D using experimental data from SE(B) tests. At the lowest temperature, -30° , the model gives an excellent prediction of the cumulative failure probability by cleavage fracture and captures the threshold toughness and the experimental scatter. At 25º  and 55º  the model slightly overestimates the fracture probability. In Paper E a serie of fracture experiments is performed on half-elliptical surface cracks at 25º in order to further verify the model. Experiments show a significant scatter in the fracture toughness. The model significantly overestimates the fracture probability for this crack geometry. / QC 20100910