This paper reviews the polarization properties of Thomson scattered (TS) light as applied to electron temperature measurement. The theoretical background is based on Stokes vector transformations and Mueller matrices. The review starts from scattering on a single electron and proceeds to the combined effect of many particles. Then, this general approach is subdivided into frequency-integrated and frequency-resolved applications. For each of them, the exact relativistic analytical solutions are presented in the form of Mueller matrix elements averaged over the relativistic Maxwellian distribution function. The dependencies of the elements on the scattering angle, electron temperature, and frequency of the scattered radiation (in the frequency-resolved case) are presented. These solutions form the basis for accurate analysis of the degree of depolarization of TS radiation. Results obtained for the frequency-integrated regime are reviewed and new solutions for the frequency-resolved case are reported, making a bridge between the two limiting cases. Experimental setups for polarization-based TS diagnostics are compared. A combination of polarization-based and spectral-based techniques are also described.