Robert Cahn demonstrated, many years ago, that purely thermal recovery at high temperatures occurs by polygonisation, the first seen example of cell formation in a dislocated crystal. Here, we now consider low temperature recovery which, because of the essential role played in it by an applied stress large enough to cause plastic yielding, is known as dynamical recovery or work softening. The dominant features, which can lead to this recovery appearing in the spectacular form of a yield drop, are the creation of cellular dislocation structures in the work hardened state, with most of the glide dislocations densely packed in the cell walls where they face a forest of other dislocations as obstacles; the back stress exerted by the obstructed dislocations on the interiors of the cells so that, even though these are soft, they are prevented from yielding until the applied stress is raised further; and the stress-driven but thermally activated cutting of the glide dislocations through the forest obstacles. The way these combine to give yield drops is discussed. (C) 2004 Kluwer Academic Publishers.