"Preface This book arose out of a need felt by the authors, whose daily work is in experimental nonlinear optics, that existing books in their field, excellent as they are, and indeed from which they learned the subject, don't always have the right formulas or derivations needed for understanding or predicting experimental results. Some examples of topics in which the authors particularly felt this need are: walk-off angle expressions for light propagating in anisotropic crystals, detailed derivations of the effective d coefficients in uniaxial, biaxial and isotropic crystals, expressions for power and energy conversion efficiencies in various three-wave mixing processes in both homogenous or layered (quasi phase matched) materials, and the effects of beam focusing on power conversion efficiencies beyond the pioneering work by Boyd and Kleinman, for non-resonant and resonant processes. This book is an attempt to be somewhat half-way between the monographs and textbooks by Bloembergen, Zernike and Midwinter, Yariv, Shen, Boyd, Powers etc. and the handbooks by Dmitriev, Gurzadyan and Nikogosyan, or by Sutherland. The derivations included here are often just the details left out as problems in textbooks. In the attempt to provide detailed derivations, the number of equations in this book has proliferated. Still the authors have kept the equations and derivations in the present format hoping that they may be as useful to some readers as they have been to them. The detailed derivations may also help the readers in catching any mistakes that may have crept into the formulas. The authors would appreciate it very much if the readers would let the publishers know of any mistakes they find in the book"-- Provided by publisher.