This book is an extended version of the proceedings of the Symposium on Polymer Composites, Interfaces, which was held under the auspices of the Division of Polymer Chemistry, American Chemical Society (ACS) during the annual ACS meeting in Seattle, March, 1983. The importance of the interface in composite materials has been recognized since the inception of modern composite technology. Specifically, silane coupling agents were developed for glass fiber reinforced compOSites at a very early date. Ever since then the diversity of composite materials and the development of various surface treatment methods have led to the establishment of an "interface art." A trial-and-error approach has dominated the interfacial aspects of composite technology until very recently. With the advent of modern analytical techniques for surface characterization, it became possible to study detailed surface and interface structures. It was hoped that this symposium would catalyze such a fundamental and scientific approach in composite studies. For this reason, the symposium was structured to verify the influence of interfacial structures on the mechanical and physical performance of composites and to improve our knowledge of the microstructure of composite interfaces. As the word *composite" indicates, interdisciplinary interaction is indispensable for proper understanding of multiphase systems.