This chapter studies spontaneous scattering of light in single-mode fibers, due to the photoelastic perturbations associated with the oscillations of guided acoustic modes. The acoustic modes, in this case, are not stimulated by the optical fields being observed. Instead, they may be of thermal origin or driven by other optical field components than those monitored. Scattering is formulated in terms of nonlinear polarization terms and nonlinear wave equations for the evolution of the spectral sidebands of an input optical field. In the case of radial guided acoustic modes, photoelastic scattering of the optical field results in its phase modulation. By contrast, torsional-radial acoustic modes may induce phase modulation, coupling to the orthogonal polarization, or a combination of both, depending on the state of polarization of the input optical field. The strength of modulation is quantified in terms of a nonlinear coefficient, with units of W−1 × m−1. The coefficient depends on acoustic frequency and the choice of mode. Spontaneous scattering by guided acoustic modes adds up with contributions of Kerr nonlinearity.