  The understanding of the large-scale structure formation requires the
resolution of coupled nonlinear equations describing the cosmic density and
velocity fields. This is a complicated problem that, for the last decade, has
been essentially addressed with N-body simulations. There is however a regime,
the so-called quasi-linear regime, for which the relative density fluctuations
are on average below unity. It is then possible to apply Perturbation Theory
techniques where the perturbation expansions are made with respect to the
initial fluctuations. I review here the major results that have been obtained
in this regime.
