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Dislocation activity in the vicinity of a crack tip and the brittle-to-ductile transition (BDT) are analysed using discrete dislocation dynamics simulations. The comparison of these simulations with fracture experiments on tungsten single crystals helps to identify the decisive mechanisms for the BDT of this material. Dislocation nucleation and the availability of active sources are shown to be limiting plasticity at low temperatures and partly in the semi-brittle regime. At elevated temperatures, fracture toughness, crack tip plasticity and the BDT itself can all be viewed as thermally activated processes, which can all be scaled by the same activation energy. It is concluded that they must be controlled by dislocation mobility