Abstract
The purpose of this chapter is to discuss two major categories of nonlinear effects related either to nonlinear refractive index or to nonlinear stimulated scattering effects. The effects related to nonlinear refractive index occur due to the dependence of the refractive index on the optical signal intensity. On the other hand, stimulated scattering effects are caused by interaction between light and material. As many wavelength signal channels travel through the optical fiber, they encounter many of those nonlinear effects impairments that affect the signal power level and hence degrade sharply the quality of the signal resulting in interference among signals carried by different wavelength channels. Therefore, before delving into the optical transmission performance issues, a physical picture of how optical signals behave in the presence of the most important nonlinear effect impairments is drawn.