 order to set up a mathematical model of nonlinear propagation, an expression has to be adopted for the nonlinear polarization. Assuming to be in the weak nonlinear propagation regime, series approximation of the nonlinear susceptibility can be adopted. In general, the relationship between the susceptibility and the field is not a local and instantaneous relationship, due to the presence of distributed interactions as in the case of Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices the Brillouin scattering. In similar cases, the susceptibility depends on the field not only in a single point and in a single instant, but incorporates a dependence extended to a finite area and to a finite time interval. Formally, this property can be coded in mathematical terms considering the polariza- tion vector as a convolution between the nonlinear susceptibility and the field [17], but the resulting integral-differential wave equation is really difficult to manage. For this reason, we will assume that all fiber nonlinearities are characterized by a scalar nonlinear susceptibility. We know that in this way we will have a unified theory of Kerr and Raman effect, but we will be unable to describe Brillouin effect. For this last case, an ad hoc model will be developed. In the local and instantaneous approximation, the Taylor series of the polarization reads (4.33) but for X(1) that represents the linear susceptibility, the generic term [x(]) of the power expansion is an Euclidean tensor with rank k+1 SO that the generic term results to be a vector, whose components are forms of degree k in the electric field components. In particular, since the glass composing the fiber is an isotropic medium at molecular level, it can be demonstrated that [x(2)] =0; thus, the nonlinear propagation is mainly due to the third term of the expansion (4.33) [7]. The tensor nature of X³ is responsible for nonlinear polarization evolution, that is, a quite weak phenomenon, but important in some kinds of long-haul systems