  Anisotropic flow is recognized as one of the main observables providing
information on the early stage of a heavy-ion collision. At RHIC the observed
strong collective flow of the bulk matter is considered evidence for an early
onset of thermalization, and an ideal hydrodynamical expansion with an equation
of state consistent with that obtained from lattice QCD calculations. The large
collective flow of the bulk and the inferred large energy loss of the produced
jets propagating through the created matter are key signatures for the
formation of a Quark Gluon Plasma.
