Polymer composites represent a field of intense and growing interest to consumers and producers of plastics. They are used to solve the most acute problems: energy and oil conservation, improvement of the properties of polymer materials, and the increase of their use. Even special problems, such as inflammability of plastics and industrial wastes, are closely connected with polymer composites. The achievements in this field are well known. Polymer composites have been widely used in building, furniture, electric appliances, cars, and other fields. Their production is growing at a higher rate than that of polymers as a whole. In the present book the emphasis is put on the principles that may become the foundation of designing new highly effective composites. The authors analyze their favorable properties as compared to "unfilled" polymers, as well as the means to improve moldability and strength. Economical and technical problems are examined. Special attention is paid to the matching of the components, development of technological processes of composites production, and to new ideas in the field. Fundamental and practical aspects of calculating properties and structure of composites are examined. The scope of the book does not include composites based on continuous reinforcing fibers, polymeric concretes, nor other special-purpose materials in which polymers are used to modify the properties of inorganic materials. The book reflects mainly ideas developed at the Institute of Chemical Physics of the USSR Academy of Sciences, but it also contains a review of the latest works in the field.