  In these proceedings, I shall review the basic concepts of perturbative QCD
in its high-energy limit, emphasising the approach to the unitarity limit,
usually referred to as {\em saturation}. I shall explain the basic framework
showing the need for saturation, first, from a simple picture of the
high-energy behaviour, then, giving a short derivation of the equation driving
this evolution. In the second part, I shall exhibit an analogy with statistical
physics and show how this allows to derive {\em geometric scaling} in QCD with
saturation. I shall finally consider the effects of gluon-number fluctuations
on this picture.
