Polymers in one form or another are basic constituents of every kind of living matter, whether plant or animal. It is only in the present century, however, that as a result of a wide range of scientific studies, their existence as a coherent group has come to be recognized and understood. Following this recognition and this understanding, the possibility has arisen of actually producing polymers by means of suitable chemical reactions. Originally these synthetic products, regarded as subsitituents for existing natural polymers such as rubber or silk, led to the introduction of a vast range of entirely new compounds in the field of plastics, rubber, and fibers, many of which have properties different from those of existing natural materials. At present, the study of polymers, though it includes the original natural polymers, tends to be dominated by these synthetic materials. It is from the industrial development of such polymers that the main stimulus to scientific research has arisen.