  The status of the physics of heavy ion collisions is reviewed based on
measurements over the past 6 years from the Relativistic Heavy Ion Collider
(RHIC) at Brookhaven National Laboratory. The dense nuclear matter produced in
Au+Au collisions with nucleon-nucleon c.m. energy $\sqrt{s_{NN}}=200$ GeV at
RHIC corresponds roughly to the density and temperature of the universe a few
microseconds after the `big-bang' and has been described as "a perfect liquid"
of quarks and gluons, rather than the gas of free quarks and gluons, ``the
quark-gluon plasma" as originally envisaged. The measurements and arguments
leading to this description will be presented.
