  The early stages of heavy ion collisions are dominated by high density
systems of gluons that carry each a small fraction $x$ of the momenta of the
colliding nucleons. A distinguishing feature of such systems is the phenomenon
of "saturation" which tames the expected growth of the gluon density as the
energy of the collision increases. The onset of saturation occurs at a
particular transverse momentum scale, the "saturation momentum", that emerges
dynamically and that marks the onset of non-linear gluon interactions. At high
energy, and for large nuclei, the saturation momentum is large compared to the
typical hadronic scale, making high density gluons amenable to a description
with weak coupling techniques. This paper reviews some of the challenges faced
in the study of such dense systems of small $x$ gluons, and of the progress
made in addressing them. The focus is on conceptual issues, and the
presentation is both pedagogical, and critical. Examples where high gluon
density could play a visible role in heavy ion collisions are briefly discussed
at the end, for illustration purpose.
