  Elliptic flow measurements are presented and discussed with emphasis on the
hydrodynamic character of the hot and dense QCD matter created in heavy ion
collisions at RHIC. Predictions from perfect fluid hydrodynamics for the
scaling of the elliptic flow coefficient $v_2$ with eccentricity, system size
and transverse energy are validated. A universal scaling for the flow of both
mesons and baryons is observed for a broad transverse kinetic energy range when
quark number scaling is employed. This suggests a new state of nuclear matter
at extremely high density and temperature whose primary constituents have the
quantum numbers of quarks and anti-quarks in chemical equilibrium. The scaled
flow is used to constrain estimates for several transport coefficients
including the sound speed $c_s$, shear viscosity to entropy ratio $\eta/s$,
diffusion coefficient ($D_c$) and sound attenuation length ($\Gamma$). The
estimated value $\eta/s \sim 0.1$, is close to the absolute lower bound
($1/4\pi$), and may signal thermodynamic trajectories for the decaying matter
which lie close to the QCD critical end point.
