It is pointed out that proposed mechanisms of fatigue fracture should be consistent with Coffin's Law for constant strain-amplitude cyclic deformation. Most proposed mechanisms are not. The fracture of dislocation cores along their glide planes is consistent with the fact that the law describes the behavior during the first 1/4 cycle as well as that after a very large number of cycles. Experimental evidences of the relative weakness of edge dislocation cores is described. Analysis of this weakness by Bullough, and in this article, discussed. It is concluded that this effect plays an important role in fatigue fracture.