Thirty years ago, Zavoisky, in Moscow (USSR), reported the first successful experi- mental observations of the ESR phenomenon. Its application to polymer problems began about 20 years ago. ESR belongs to the most specific and useful methods in the study of polymer reactions. The main purpose of this book is to collect the present available information on the applications of electron spin resonance (ESR) spectroscopy in polymer research. The book has been written both for those who want an introduction to this field, and for those who are aheady familiar with ESR and are interested in application to polymers. Therefore, the fundamental principles of ESR spectroscopy are first out- lined, the experimental methods including computer applications are described in more detail, and the main emphasis is on the application of ESR methods to polymer problems. Many results obtained are only briefly treated for lack of space. The authors hope that this book will provide a useful source of information by giving a coherent treatment and extensive references to original papers, reviews, and discussions in monographs and books. In this way we hope to encourage polymer chemists, organic chemists, biochemists, physicists, and material scientists to apply ESR methods to their research problems.