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Quark-gluon plasma creation in relativistic heavy-ion collisions requires that the reaction zone thermalizes before the energy density drops below the critical value for deconfinement. Thermalization generates pressure, and pressure generates collective flow. I discuss two types of flow, radial and elliptic flow. Elliptic flow is shown to be sensitive to pressure build-up during the early collision stages. New data from Au+Au collisions at RHIC are in quantitative agreement with hydrodynamic calculations which assume almost instantaneous thermalization. They badly disagree with microscopic transport models employing incoherent scattering between on-shell partons and/or hadrons. The data thus give evidence for unexpectedly rapid kinetic equilibration and provide a serious challenge for our present understanding of the microscopic collision dynamics.
