  Is gravitational growth responsible for the observed large scale structure in
the universe? Do we need non-gaussian initial conditions or non-gravitational
physics to explain the large scale features traced by galaxy surveys? I will
briefly revise the basic ideas of non-linear perturbation theory (PT) as a tool
to understand structure formation, in particular through the study of higher
order statistics, like the skewness and the 3-point function. Contrary to what
happens with the second order statistics (the variance or power-spectrum), this
test of gravitational instability is independent of the overall amplitude of
fluctuations and of cosmic evolution, so that it does not require comparing the
clustering at different redshifts. Predictions from weakly non-linear PT have
been compared with observations to place constraints on our assumptions about
structure formation, the initial conditions and how galaxies trace the mass.
