Heavy Ion Physics at RHIC

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.