  A newly proposed framework of perfect-fluid relativistic hydrodynamics for
particles with spin 1/2 is briefly reviewed. The hydrodynamic equations follow
entirely from the conservation laws for energy, momentum, and angular momentum.
The incorporation of the angular-momentum conservation requires that the spin
polarization tensor is introduced. It plays a role of a Lagrange multiplier
conjugated to the spin tensor. The space-time evolution of the spin
polarization tensor depends on the specific form chosen for the spin tensor.
