  One of the most striking results from the Relativistic Heavy Ion Collider is
the strong elliptic flow. This review summarizes what is observed and how these
results are combined with reasonable theoretical assumptions to estimate the
shear viscosity of QCD near the phase transition. A data comparison with
viscous hydrodynamics and kinetic theory calculations indicates that the shear
viscosity to entropy ratio is surprisingly small, $\eta/s < 0.4$. The preferred
range is $\eta/s \simeq (1\leftrightarrow 3) \times 1/4\pi$.
