  Since the first evidence of a global polarization of $\Lambda$ hyperons in
relativistic nuclear collisions in 2017, spin has opened a new window in the
field, both at experimental and theoretical level, and an exciting perspective.
The current state of the field is reviewed with regard to the theoretical
understanding of the data, reporting on the most recent achievements and
envisioning possible developments. The intriguing connections of spin physics
in relativistic matter with fundamental questions in quantum field theory and
applications in the non-relativistic domain are discussed.
