It is now well established that a unique feature of coherent optical beams is their ability to transmit, process, store and interconnect in parallel a large number of high bandwidth information channels. However, although these techniques possess great potential their development depends critically on the nonlinear optical effects used and on the availability of nonlinear optical materials that work at high speed and low incident optical power. At present, these requirements are stimulating a great deal of research in materials science and are challenging existing technologies, in particular high speed electronics. This volume devoted to nonlinear optical effects and materials presents a detailed account of selected topics in inorganic and organic materials re- search. The status of organic crystals and polymers for nonlinear optics is critically compared with their inorganic counterparts. The preparation tech- niques and a description of the methods used to characterize the nonlinear optical effects relevant for device applications are dealt with, as well as a theoretical description of the nonlinear optical, electro-optical and photore- fractive effects observed. The main concepts and potential applications are outlined and developed in the various chapters of this book. This collection of articles provides a broad survey of selected research topics in organic and in- organic nonlinear optics.