  Heavy ion collisions at RHIC energies (Au+Au collisions at
$\sqrt{s_{NN}}=200$ GeV) exhibit significant new features as compared to
earlier experiments at lower energies. The reaction is characterized by a high
degree of transparency of the collisions partners leading to the formation of a
baryon-poor central region. In this zone, particle production occurs mainly
from the stretching of the color field. The initial energy density is well
above the one considered necessary for the formation of the Quark Gluon Plasma,
QGP. The production of charged particles of various masses is consistent with
chemical and thermal equilibrium. Recently, a suppression of the high
transverse momentum component of hadron spectra has been observed in central
Au+Au collisions. This can be explained by the energy loss experienced by
leading partons in a medium with a high density of unscreened color charges. In
contrast, such high $p_t$ jets are not suppressed in d+Au collisions suggesting
that the high $p_t$ suppression is not due to initial state effects in the
ultrarelativistic colliding nuclei.
