In the present Chapter, the background and fundamentals of the nonlinear optical materials in the perspective of the all-optical switching applications. The interaction of the intense laser pulses with the materials is described which is followed by the introduction to the second and the third-order nonlinear optical processes. The contents of this Chapter can be easily found in the other standard books of lasers and nonlinear optics although a special emphasis is given to the applications of the third-order nonlinear susceptibility in the field of all-optical switching. Various topics such as origin of the nonlinear optical response, electronic response, slowly varying amplitude approximation for degenerate nonlinear refraction and nonlinear absorption, molecular reorientation response are elaborated at length.