2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/219794I 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.Based on the Lecture Course given at the Les Houches Summer School on `Long Range Interacting Systems', 4-29 August 2008; 25 pages; 5 figures; ver 2 typo in author's name corrected and one reference addedAstrophysicstext