This chapter deals with a more detailed comparison of the glass–liquid and crystal–liquid liquid transitions, focusing on the temperature dependence of the specific volume. It describes several approaches to characterizing the glass transition in polymers. The molecular mechanisms of plastic deformation and failure in glassy polymers are far from completely understood, but some important basic principles have been elucidated. In glassy polymers the lack of structural regularity, combined with the substantial spatial extent and interpenetration of the covalently bonded chains, makes the mechanical response more uniform. Cracks in polymers, therefore, are usually initiated by defects on the surface, such as scratches. The mode of crack propagation after initiation is also qualitatively different in polymers than in metals and ceramics; the new phenomenon is called crazing. The regularity of the microstructure of the polymer molecule, along with experimental conditions, determines the extent of crystallization.