  After 15 years of heavy-ion collision experiments at the AGS and SPS, the
recent turn-on of RHIC has initiated a new stage of quark-gluon plasma studies.
I review the evidence for deconfined quark-gluon matter at SPS energies and the
recent confirmation of some of the key ideas by the new RHIC data. Measurements
of the elliptic flow at RHIC provide strong evidence for efficient
thermalization during the very early partonic collision stage, resulting in a
well-developed quark-gluon plasma with almost ideal fluid-dynamical collective
behaviour and a lifetime of several fm/c.
