We present a theoretical study of multi-mode scattering of light by optically random media, using the Mueller-Stokes formalism which permits to encode all the polarization properties of the scattering medium in a real 4×4 matrix. From this matrix two relevant parameters can be extracted: the depolarizing power DM and the polarization entropy EM of the scattering medium. By studying the relation between EM and DM , we find that all scattering media must satisfy some universal constraints. These constraints apply to both classical and quantum scattering processes. The results obtained here may be especially relevant for quantum communication applications, where depolarization is synonymous with decoherence.PACS numbers: 03.65. Nk, 42.25.Dd, 42.25.Fx, 42.25.Ja Introduction The polarization aspects of random light scattering have drawn quite some interest in recent years, since they present a diagnostic method of the medium involved and also help visualization of objects that are hidden inside the medium [1]. When polarized light is incident on an optically random medium it suffers multiple scattering and, as a result, it may emerge partly or completely depolarized. The amount of depolarization can be quantified by calculating either the entropy (E F ) or the degree of polarization (P F ) of the scattered field [2]. It is simple to show that the field quantities E F and P F are related by a single-valued function: E F = E F (P F ). For example polarized light (P F = 1) has E F = 0 while partially polarized light (0 ≤ P F < 1) has 1 ≥ E F > 0. When the incident beam is polarized and the output beam is partially polarized, the medium is said to be depolarizing. An average measure of the depolarizing power of the medium is given by the so called depolarization index (D M ) [3]. Non-depolarizing media are characterized by D M = 1, while depolarizing media have 0 ≤ D M < 1. A depolarizing scattering process is always accompanied by an increase of the entropy of the light, the increase being due to the interaction of the field with the medium. An average measure of the entropy that a given random medium can add to the entropy of the incident light beam, is given by the polarization entropy E M [4]. Non-depolarizing media are characterized by E M = 0, while for depolarizing media 0 < E M ≤ 1. As the field quantities E F and P F are related to each other, so are the medium quantities E M and D M with the key difference that, as we shall show later, E M is a multi-valued function of D M .