One of the important aims of modern polymer research is to achieve an understanding of molecular chain conformation and of the spatial arrangement of the polymer molecules in the solid state. In attempts to solve this problem and to interpret the physical properties of high polymers, it is necessary to remember that a polymer structure must be considered as dynamic rather than as static. Due to its constitution, the structural system of a bulk polymer is not stable but goes toward its thermodynamic equilibrium by the conformational motions of chain elements changing the arrangement of macromolecules. The rate and spatial reach of molecular motions depend to a great extent on the temperature and, in fact, define the number of thermal transitions observed in polymers. The two most important thermal transitions are the glass-rubber transition, characterized by the temperatureTg, and the melting of crystalline elements, characterized by the temperature T, (in semicrystalline polymers). BelowTgthe material is in its hard glassy state, and molecular motions are restricted to side chains or to slow, short-range conformational movements of the amorphous phase. At higher temperatures, aboveTg, which is the onset of segmental mobility of backbone chains in the amorphous regions, the material is in a soft rubbery state. Increased segmental mobility allows certain structural transformations; for example, increased chain packing and a crystallite growth. AboveTm, crystallites are molten and the whole material is in a liquid state of high viscosity.