  The dynamical evolution of collisionless particles in an expanding background
is described. After discussing qualitatively the key features, the
gravitational clustering of collisionless particles in an expanding universe is
modelled using some simple physical ideas. I show that it is indeed possible to
understand the nonlinear clustering in terms of three well defined regimes: (1)
linear regime (2) quasilinear regime which is dominated by scale-invariant
radial infall and (3) nonlinear regime dominated by nonradial motions and
mergers. Modelling each of these regimes separately I show how the nonlinear
two point correlation function can be related to the linear correlation
function in hierarchical models. This analysis leads to results which are in
good agreement with numerical simulations thereby providing an explanation for
numerical results. The ideas presented here will also serve as a powerful
analytical tool to investigate nonlinear clustering in different models.
Several implications of the result are discussed.
