  Heavy ion collisions generate strong fluid vorticty in the produced hot
quark-gluon matter which could in turn induce measurable spin polarization of
hadrons. We review recent progress on the vorticity formation and spin
polarization in heavy ion collisions with transport models. We present an
introduction to the fluid vorticity in non-relativistic and relativistic
hydrodynamics and address various properties of the vorticity formed in heavy
ion collisions. We discuss the spin polarization in a vortical fluid using the
Wigner function formalism in which we derive the freeze-out formula for the
spin polarization. Finally we give a brief overview of recent theoretical
results for both the global and local spin polarization of $\Lambda$ and
$\bar\Lambda$ hyperons.
