Four years ago when we had finished our work on the first edition of our Handbook we didn’t even suppose that three years later it would become necessary to greatly revise and update the material. It happened because of the following developments. 1. The invention and tremendous development of modern nonlinear optical crystals such as BBO, LBO, KTP, Zn Ge P , etc. 2 2. Rapid progress in laser techniques (femtosecond CPM laser, Ti: sapphire laser, diode-pumped solid-state lasers, etc.). 3. The appearence of numerous organic crystals which can be synthesized with predictable properties. 4. Progress in the theory of nonlinear frequency conversion utilizing biaxial crystals, femtosecond pulses, etc. 5. Accumulation of new data on the properties of nonlinear optical crystals. In accordance with the above, we have made many changes in the text. The first chapter was revised by D.N. Nikogosyan, the second one by V.G. Dmitriev and D.N. Nikogosyan, and the fourth one by G.G. Gurzadyan. The third chapter, containing the main reference material on 77 nonlinear optical crystals was completely rewritten and updated by D.N. Nikogosyan. The Appendix containing the list of most commonly used laser wavelengths was compiled by D.N. Nikogosyan. We would appreciate any valuable comments and recommendations that will allow us to further improve the Handbook. We would like to thank H.K.V. Lotsch for fruitful and long-lasting cooperation.