  Here I review measurements of v2, the second component in a Fourier
decomposition of the azimuthal dependence of particle production relative to
the reaction plane in heavy-ion collisions. v2 is an observable central to the
interpretation of the subsequent expansion of heavy-ion collisions. Its large
value indicates significant space-momentum correlations, consistent with the
rapid expansion of a strongly interacting Quark Gluon Plasma. Data is reviewed
for collision energies from sqrt[sNN] = 2 to 200 GeV. Scaling observations and
comparisons to hydrodynamic models are discussed.
