In almost all metals and alloys, dislocations are concentrated to narrow regions after plastic deformation that divide the material into cells or subgrains. The cell walls consist of tangles whereas the subgrains are surrounded by thin regular networks of dislocations. The cells are transferred to subgrains with increasing temperature. Although these substructures have been analyzed for many years, basic models of their development have only appeared recently. Models for substructures are presented for plastic deformation at constant stress and at constant strain rate. During straining the dislocations can move in opposite directions creating a polarized structure, where the possibility for recovery of dislocations is reduced. This can be expressed in term of a back stress. Its presence explains why creep curves at near ambient temperatures could have an appearance that is similar to that at elevated temperatures. It is also the basis for the effect of cold work on creep. The models can quantitatively describe why the creep rate can be reduced by up to six orders of magnitude for Cu after cold work.