Abstract Available experimental evidence on cleavage of single crystals suggests that the direct interactions of crack tips with pre-existing dislocations are important in determining the toughness of cleavable crystals. On this basis, the behavior of dislocations in the vicinity of a moving crack tip has been investigated. It is found that dislocations which reside within a strip of certain width along the crack path and which have Burgers vectors of a certain orientation are strongly attracted to the crack tip. and the motion of such dislocations with respect to the moving crack tip is determined. Once drawn into the immediate vicinity of the crack tip, these dislocations inevitably cause local, atomic scale blunting of the tip. The resulting crack trapping mechanism is modeled phenomenologically, yielding a quantitative estimate for actual crack tip fracture toughness Γ tip which depends on temperature, crack speed and density of pre-existing dislocations. This fracture toughness is believed to represent an estimate of the resistance to fracture that is more realistic than the ideal Griffith surface energy estimate. Then, possible scenarios for either gradual or abrupt transitions from brittle to ductile behavior with temperature are discussed within a framework based on the present model and a continuum plasticity model for the functional relation between the measurable far field toughness Γ far and the crack tip fracture toughness. The nature of the transition depends on the relationship between these two functions. As an application, the anomalous low temperature toughness of iron is explained on the basis of the high density of dislocations commonly observed in annealed iron crystals.