The main goal of this work is the study of cold quark matter in neutron stars from phase transitions of hadronic matter to quark matter. In this work, after presenting important aspects of stellar evolution and the theory of neutron stars, we review two frameworks designed to calculate the structure of the stars in the relativistic regime: the canonical Tolman-Oppenheimer-Volkoff equations and the so called ‘new paradigm’ Einstein-Maxwell-Fermi, where we conclude that the Einstein-Maxwell-Fermi model has some inconsistencies, being inadequate for the treatment of stars with hybrid equation of state, and the fact that using the atomic model as a starting point already has the property of the local non-neutrality as a built-in feature.