Polymer blends, grafts, and blocks, broadly defined, encompass all of the ways in which two or more kinds of poly­ mer molecules can be mixed and/or joined. Because these mate­ rials exhibit non-linear and often synergistic properties, they have found increasing application in our technology. Their multifarious uses have, in turn, spurred new research efforts, to find yet different ways of joining two kinds of polymer molecules, with novel physical and/or mechanical behavior patterns. In August, 1973, the Polymer Division of the American Chemical Society sponsored a symposium at its meeting in Chi­ cago on Polymer Blends, Grafts, and Blocks. This book collects the papers presented at that symposium. Yet, it is more than just a collection of papers, for we here display the thinking and efforts of a number of top-ranking American and foreign scientists in one of the world's more active research areas. The symposium emphasized the interrelationships among synthetic detail, morphology, and physical and mechanical properties. Several novel syntheses were presented. These include oxidation resistant thermoplastic elastomers (Holden), a graft copolymer based thermoplastic elastomer (Kennedy and Smith), a cationic graft copolymer (Kennedy, Charles, and Davidson), an AB crosslinked copolymer (Bamford and Eastmond), an interpenetrating polymer network (Donatelli, Thomas, and Sperling), and simultaneous interpenetrating networks (Frisch, Klempner, Frisch, and Ghiradella). Most polymer blends, grafts, and blocks exhibit two phases. The theory of microdomain structure was discussed (Helfand). The different ways that the two molecules can be joined together was examined (Kenney), and their topology was explored (Sperling).