Dislocation segments with Burgers vector b = are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b = 1/2. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight edge dislocation is investigated here by atomic-scale computer simulation for a-iron using three different interatomic potentials. At low temperature the dislocation has a nonplanar core consisting of two 1/2 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the dislocation. It is concluded that the response of the edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.