  This paper covers the material of our two talks. We describe a series of
projects based upon perturbative expansions to follow the gravitational
evolution of the one point probability distribution functions (PDFs) for the
density contrast field and for the divergence of the corresponding velocity
field. The Edgeworth expansion greatly simplifies the problem. Indeed, the PDF
can be described through several low order moments, with the first non-trivial
contribution coming from the skewness, or third moment. Gaussian initial
conditions imply that the skewness of the smoothed density field is
proportional to the square of the variance. The constant of proportionality is
then a function of the logarithmic slope of the variance versus scale, and is
otherwise independent of scale. This constant is also practically independent
of the density parameter $\Omega$, and has very nearly the same value when
distances are estimated by redshifts. To show the latter property, we also
briefly discuss our Lagrangian approach to perturbation theory. Finally, the
observed skewness does depend, of course, on the relation between the
distribution of light and mass; we describe the case of local biasing. For the
divergence of the velocity field, the skewness of its PDF is also proportional
to the square of its variance, but the ratio now strongly depends on the value
of $\Omega$. This offers a new method to determine the density parameter which
does not involve comparisons
