The mechanical work spent on the irreversible deformation of solids is always at least partly dissipated. Therefore, independent of the sign of the heat effect resulting from the elastic deformation, plastic deformation is always accompanied by an exothermic effect. A schematic diagram of thermo-mechanical behavior of an ordinary solid and a solid polymer is shown in Fig. 7.1. After a small amount of initial cooling, resulting from elastic deformation, evolution of heat accompanying the beginning of plastic deformation occurs. The appearence of plastic deformation is accompanied by heat evolution independent of whether it is localized (necking) or distributed along the sample uniformly. If the plastic deformation is accompanied with a neck formation, which is typical of the cold drawing of the majority of glassy and crystalline polymers well below glass transition or melting point, then the heat generated locally may lead to a considerable local temperature rise. This temperature rise may strongly influence the cold drawing of the sample. At low drawing rates the neck propagates uniformly along the sample. Under certain conditions, however, especially at a high rate of extension for some polymers, instabilities of neck propagation can be observed (self-oscillation phenomenon), which is also closely related with the local thermal effects.