In recent years the development of the NMR method has been closely linked to the creation of the theory of the NMR in solids and with the elaboration on their basic principles and methods of high resolution in solids. The aim of this volume is to explain simply and clearly the principles for the understanding of the NMR in solid polymers, to show what kind of information can be gained using NMR investigations of bulk polymers, and to demonstrate the ways of obtaining this information, their correctness and their place in comparison with other methods. The discussion is restricted to the most important and typical nuclei 1H, 13C, and (briefly) 2D, and, in connection with that, to the dipolar and quadrupolar interaction and to the chemical shift.