  Chiral symmetry represents a fundamental concept lying at the core of
particle and nuclear physics. Its spontaneous breaking in vacuum can be
exploited to distinguish chiral hadronic partners, whose masses differ. In
fact, the features of this breaking serve as guiding principles for the
construction of effective approaches of QCD at low energies, e.g., the chiral
perturbation theory, the linear sigma model, the
(Polyakov)--Nambu--Jona-Lasinio model, etc. At high temperatures/densities
chiral symmetry can be restored bringing the chiral partners to be nearly
degenerated in mass. At vanishing baryochemical potential, such restoration
follows a smooth transition, and the chiral companions reach this degeneration
above the transition temperature. In this work I review how different
realizations of chiral partner degeneracy arise in different effective
theories/models of QCD. I distinguish the cases where the chiral states are
either fundamental degrees of freedom or (dynamically-generated) composed
states. In particular, I discuss the intriguing case in which chiral symmetry
restoration involves more than two chiral partners, recently addressed in the
literature.
