  We describe the current status of the heavy ion research program at the
Relativistic Heavy Ion Collider (RHIC). The new suite of experiments and the
collider energies have opened up new probes of the medium created in the
collisions. Our review focuses on the experimental discoveries to date at RHIC
and their interpretation in the light of our present theoretical understanding
of the dynamics of relativistic heavy ion collisions and of the structure of
strongly interacting matter at high energy density.
