is the statement that the number of partons per unit of transverse phase space in the Fock states of the incoming hadrons does not grow exponentially forever with energy (or rapidity), as would come out of a naive solution of linear evolution equations: The growth gets softer at very high energies, in such a way that the unitarity of the probabilities of scattering be preserved.

However, it seems very difficult to get saturation from a field-theoretical calculation. So far, the problem has not even been formulated properly in QCD, for it is already very challenging to identify the graphs that should be taken into account. Equations such as the Balitsky equation [@Balitsky:1995ub] (which is in fact a hierarchy of equations) have been written down, but they do not exhibit saturation in an obvious way (if at all), and they are anyway extremely difficult to solve.

Despite these fundamental difficulties, new quantitative results could be obtained for QCD amplitudes in the saturation regime over the last few years [@Mueller:2004sea; @IMM]. In short, they rely on an analogy with reaction-diffusion processes [@IMM] but not on the computation of definite Feynman graphs. The obtained results do not depend on the way how partons 