QCD at High Energy: Saturation and Fluctuation Effects

In this article, I shall review the basic concepts of perturbative QCD in its high-energy limit. I shall concentrate on the approach to the unitarity limit, usually referred to as saturation, as well as on the gluon-number fluctuations the importance of which has recently been discovered. I shall explain the basic framework showing the need for those phenomena, st, 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 geometric scaling in QCD with saturation. I shall ally consider the effects of gluon-number fluctuations on this picture and draw the physical consequences, i.e. a new scaling law, arising from those results. (author)