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 Brook haven National Laboratory. The dense nuclear matter produced in Au + Au collisions with nucleon-nucleon c.m. energy root(NN)-N-s = 200 GeV at RHIC corresponds roughly to the density and temperature of the universea few microseconds after the 'big-bang' and has been described as a perfect liquid of quarks and gluons, rather than the gas of freequarks and gluons, the quark-gluon plasma as originally envisaged. The measurements and arguments leading to this description will be presented.