About ten years after the publication of the Second Edition (1973), it became apparent that it was time for an up-date of this book. This was especially true in this case, since the subject matter has traditionally dealt mainly with the structure, properties, and technology of the various elastomers used in industry, and these are bound to undergo significant changes over the period of a decade. In revising the contents of this volume, it was thought best to keep the orig inal format. Hence the first five chapters discuss the same general subject matter as before. The chapters dealing with natural rubber and the synthetic elastomers are up-dated, and an entirely new chapter has been added on the thermoplastic elastomers, which have, of course, grown tremendously in importance. Another innovation is the addition of a new chapter, "Miscellaneous Elastomers," to take care of "old" elastomers, e.g., polysulfides, which have decreased some what in importance, as well as to introduce some of the newly-developed syn thetic rubbers which have not yet reached high production levels. The editor wishes to express his sincere appreciation to all the contributors, without whose close cooperation this task would have been impossible. He would especially like to acknowledge the invaluable assistance of Dr. Howard Stephens in the planning of this book, and for his suggestion of suitable authors.