The book "Nonlinear Optics" provides a comprehensive overview of the latest developments in the field of nonlinear optics. It combines mathematical derivations with experimental verifications and highlights potential future applications, such as all-optical switches. The use of a practical unit system ensures a consistent and understandable presentation of the concepts. Fundamental principles of nonlinear optical effects are systematically explained through simple physical images, such as light waves and photons. The book addresses both the nonlinear wave equation and the interactions between light and electric dipoles, supplemented by the application of rate equations based on the energy transitions of particle systems. It is aimed at researchers, engineers, and students in the fields of optics, optoelectronics, fiber communication, information technology, and materials science.