  A great effort has been devoted to formulate a classical relativistic theory
of spin compatible with quantum relativistic wave equations. The main
difficulty in order to connect classical and quantum theories rests in finding
a parameter which plays the role of proper time at a purely quantum level. We
present a partial review of several proposals of classical and quantum spin
theories from the pioneer works of Thomas and Frenkel, revisited in the
classical BMT work, to the semiclassical model of Barut and Zanghi [Phys. Rev.
Lett. 52, 2009 (1984)]. We show that the last model can be obtained from a
semiclassical limit of the Feynman proper time parametrization of the Dirac
equation. At the quantum level we derive spin precession equations in the
Heisenberg picture. Analogies and differences with respect to classical
theories are discussed in detail.
