This chapter presents a consideration of the fundamental features of the polarization decay in the course of propagation in multiple scattering randomly inhomogeneous media. The concept of similarity between various multiple scattering phenomena (temporal decorrelation of laser radiation in non-stationary random media, decrease in the intensity of propagating light in absorbing multiple scattering media, decrease in the polarization degree relative to the initial state during propagation) is substantiated. In the framework of this concept, the decay in the polarization degree of propagating light can be described using a linear integral transform of the probability density function of propagation paths along which partial components of the scattered field travel in the medium. This probability density function is defined for used illumination and detection conditions, and the kernel of the integral transform, in accordance with numerous empirical and model data, can be represented in exponential form. Thus, the assessment of the degree of residual polarization in multiple scattering with given illumination and detection conditions can be reduced to calculating the one-sided Laplace transform. The characteristic spatial scale of polarization decay in a medium, included in the exponential kernel, or depolarization length, is determined by the illumination and detection conditions, as well as the optical transport parameters of the medium and the wavelength of the probing radiation.