This chapter discusses the nonlinear optics in composite materials. The chapter presents the different experimental and theoretical results relating to nonlinear optical properties of these materials. The preparation techniques of these materials, and the linear and nonlinear optical characterization techniques are discussed. The chapter outlines electronic motion and its coupling. The nonlinear optical properties of metal-doped glasses and colloids, and the semiconductor-doped glasses and colloids are discussed. The effects of quantum and dielectric confinements are quantum-mechanical and classical, respectively. The several drawbacks of the composite materials still compare favorably with the homogeneous nonlinear materials organics or inorganics. In addition, they show properties that are not shared by the homogeneous materials, such as robustness, adaptability, thermal resistance, and many others, and most importantly, their properties can be artificially tailored through the confinement effects to meet many demands. © 1991, Elsevier B.V. All rights reserved.