  Collisions of heavy nuclei at very high energies offer the exciting
possibility of experimentally exploring the phase transformation from hadronic
to partonic degrees of freedom which is predicted to occur at several times
normal nuclear density and/or for temperatures in excess of $\sim 170$ MeV.
Such a state, often referred to as a quark-gluon plasma, is thought to have
been the dominant form of matter in the universe in the first few microseconds
after the Big Bang. Data from the first five years of heavy ion collisions of
Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) clearly
demonstrate that these very high temperatures and densities have been achieved.
While there are strong suggestions of the role of quark degrees of freedom in
determining the final-state distributions of the produced matter, there is also
compelling evidence that the matter does {\em not} behave as a quasi-ideal
state of free quarks and gluons. Rather, its behavior is that of a dense fluid
with very low kinematic viscosity exhibiting strong hydrodynamic flow and
nearly complete absorption of high momentum probes. The current status of the
RHIC experimental studies is presented, with a special emphasis on the fluid
properties of the created matter, which may in fact be the most perfect fluid
ever studied in the laboratory.
