  I review several issues related to statistical description of gravitating
systems in both static and expanding backgrounds. After briefly reviewing the
results for the static background, I concentrate on gravitational clustering of
collisionless particles in an expanding universe. In particular, I describe (a)
how the non linear mode-mode coupling transfers power from one scale to another
in the Fourier space if the initial power spectrum is sharply peaked at a given
scale and (b) the asymptotic characteristics of gravitational clustering which
are independent of the initial conditions. Numerical simulations as well as
analytic work shows that power transfer leads to a universal power spectrum at
late times, somewhat reminiscent of the existence of Kolmogorov spectrum in
fluid turbulence.
