  In these proceedings I discuss the recent progress in the theory of spin
polarization in relativistic fluids. To date, a number of studies have begun to
examine the impact of the shear tensor on the local spin polarization and
whether this contribution can restore agreement between the measurements and
the predictions obtained from a polarization induced by the gradients of the
plasma. I present the derivation of the spin polarization vector of a fermion
at local thermal equilibrium and I discuss the role of pseudo-gauge
transformations and of dissipative effects. I list what we can learn from the
polarization measured at lower energies. Finally, I discuss possible
applications of spin polarization measurements in relativistic heavy ion
collisions.
