In the past ten years the science of Polyconjugated Organic Materials has grown rapidly and is now experiencing the uncorrelated explosive development typical of a new science. The transfer of the basic scientific knowledge of these materials to the field of technology and industry is presently the focus of interest in academic and industrial circles. New devices are being developed which are paving the way for future technologies. Organic materials have become the focus of attention in these technologies. The large and very fast nonlinear optical response of organic molecules has generated new theoretical and experimental physics as well as new synthetic chemistry. The advancement of knowledge and the new achievements in this field require the interdisciplinary practice of chemists, physicists and engineers who can talk the same technical language on molecular systems which show specific physical properties. The purpose of this book is to introduce beginners to the field of nonlinear optics in organic materials and to expose specialists in one field to the problems of the other fields. Since organic molecules with a large and very fast nonlinear optical response are being continuously discovered the contributions focus on this class of materials. The volume provides a useful introduction for all those interested in the theoretical and experimental aspects of this expanding field.