  The concepts and methods used for the study of disordered systems have proven
useful in the analysis of the evolution equations of quantum chromodynamics in
the high-energy regime: Indeed, parton branching in the semi-classical
approximation relevant at high energies is a peculiar branching-diffusion
process, and parton branching supplemented by saturation effects (such as gluon
recombination) is a reaction-diffusion process. In these lectures, we first
introduce the basic concepts in the context of simple toy models, we study the
properties of the latter, and show how the results obtained for the simple
models may be taken over to quantum chromodynamics.
