Nicolaas Bloembergen, recipient of the Nobel Prize for Physics (1981), wrote "Nonlinear Optics" in 1964, when the field of nonlinear optics was only three years old. The available literature has since grown by at least three orders of magnitude. The vitality of "Nonlinear Optics" is evident from the still-growing number of scientists and engineers engaged in the study of new nonlinear phenomena and in the development of new nonlinear devices in the field of opto-electronics. This monograph should be helpful in providing an historical introduction and a general background of basic ideas both for experts specializing in this discipline and for scientists and students who wish to become acquainted with it. This is the fourth reprint and includes new references to the recent literature.