Nonlinear optics is attracting attention because of its applications in telecommunications and its possibilities for optical computing. Optoelectronics has been singled out by many as being of considerable strategic importance. Optical fibre communications shows that optics is already a method of choice; nonlinear optics offers further improvements in efficiency and versatility. Textbooks on nonlinear optics are readily available, concentrating on phenomena and devices rather than on the materials. Materials for nonlinear optics have developed considerably, with novel semiconductor structures and organics attracting much attention. These too are treated in a variety of books, at a research level. To take advantage of these newer materials, those who use nonlinear optics may need a source of information on the materials available and their special characteristics. At the same time, materials specialists entering the field of nonlinear optics may need a source of information on the general features of nonlinear optics. Nonlinear optics also now forms part of a number of degree and Masters level programmes. This book is written as an overview of the subject for users of these materials, graduate level courses and those entering research in this area. Chapters cover individual classes of materials and try to provide an authoritative but approachable summary of the characteristics of each class, with emphasis on the advantages offered for applications in nonlinear optics. To ensure that the book is accessible to those not conversant with nonlinear optics, introductory chapters outline the principle nonlinear optical phenomena and how these are exploited in device configurations.