  High energetic particles traversing a dense medium lose a sizable part of
their energy in form of gluon radiation. As a result, the rate of high-$p_t$
particles is expected to be suppressed in heavy ion collisions with respect to
the proton case. Recent experimental data from RHIC strongly support this
scenario. This allows to study the properties of the medium by the amount of
jet quenching it produces. The angular dependence of the radiation is modified
in the medium in a characteristic way. This provides another tool to study the
medium properties in a more differential measurement.
