Over the course of past decade, important progress has been made in kesterite based Cu2Zn(SnxGe1-x)(SySe1-y)4 (CZTGSSe) thin film solar cells that show excellent properties as absorber material such as an optimal and tuneable band gap. Other advantages of the kesterites are related to the fact that they are constituted by low–cost elements with low toxicity and abundant in the Earth`s crust, which allows avoiding the use of critical raw materials. The best solar cell efficiency of 9 % for pure CZTS based solar cells and of 12.6 % for the alloyed kesterite photovoltaic devices has been reported. Other possible applications of CZTS range from utilization as a wide–gap p–type thermoelectric material.  Despite of the considerable efforts aimed at the development of kesterite thin film technology and the photovoltaic conversion improvements, there is still a limited number of works centred on the analysis of their fundamental material properties. However, for the optoelectronic applications it is mandatory to have a deeper and more accurate knowledge on the optical properties in a wide photon energy range. The spectroscopic ellipsometry (SE) is a key technique to study the pseudodielectric function, ε, which is the basis of optical parameters supplying, besides, the transition energies of critical points [1].  In this work we review the optical study of kesterites starting from the theoretical approach used to calculate the DOS, the ε() and the () [2]. Continuing with the theory used to model the ellipsometric measurements; mainly we compare the results for two and three phase models. Finally, we discuss the influence of the models in the optical parameters and transitions energy values, proposing a theoretical interpretation for the experimental results [1, 3].