[en] The brittle-ductile transition (BDT) in precracked Si and other pure covalent crystals is associated with plastic relaxation processes induced by the crack tip stress field. A model which simulates the dynamic behaviour of shielding dislocations near a crack tip under an increasing stress has been developed. Dislocations generated near the transition temperature, T_c, are highly stressed, and are in a continuously developing inverse pile-up. In silicon and germanium, where the stress/temperature/velocity relations for the test materials are known, the model allows accurate prediction of the transition temperature and the ''shape'' of the BDT; we have performed experiments to confirm these predictions. We have been modelling the brittle-ductile transition in semi-brittle materials (bcc metals and oxides), where pre-existing dislocation sources appear to control the transition behaviour via slow, stable crack growth. The experimental results are described, and the characteristics and predictions of the model discussed. (orig.)