The goal of this book is to give a physical insight about the various nonlinear effects associated with the optical fiber, with a particular emphasize on Modulational Instability (MI) and Supercontinuum generation (SCG). The interaction between natural phenomena such as linear dispersion due to the frequency-dependent refractive index and the nonlinearity due to the intensity-dependent refractive index generates interesting features which has aroused a great deal of interest and pave the way for the discovery of ¿Soliton¿. MI is a phenomena in which a weak perturbation imposed on a continuous wave leads to the exponential growth of the perturbation as a consequence of the interplay between linear and nonlinear e¿ects. Another manifestation of nonlinear optical effects, which has deserved substantial interest across the photonics community, is the so-called supercontinuum generation. SCG is identi¿ed as one of the most advanced frontiers of nonlinear optics and it has been recognized by the scientific world as ¿White-Light Laser¿. Our viewpoint in this book is particularly on the in¿uence of nonlinear saturation and relaxation in the dynamics of optical pulse propagation.