Includes bibliographical references and index.

Optical nonlinearity emerges from nonlinear interaction of light with matter. In this chapter, the basic concept and formulation of light-matter interaction are discussed through a semiclassical approach. In this semiclassical treatment of the interaction between an optical field and a material, the behavior of the optical field is classically described by Maxwell's equations, but the state of the material is quantum mechanically described by a wave function that is governed by the Hamiltonian of the material.