  Can local fluctuations of a ``Quintessence'' scalar field play a dynamical
role in the gravitational clustering and cosmic structure formation process? We
address this question in the general framework of scalar-tensor theories of
gravity. Non-linear energy density perturbations, both in the scalar field and
matter component, and linear metric perturbations are accounted for in the
perturbed Einstein's equations. We derive the Newtonian limit of the relevant
equations for clustering in scalar-tensor cosmologies. We then specialize to
non-linear perturbations of the ``Extended Quintessence'' model of Dark Energy;
in such a model, a non-minimally coupled scalar field is thought to be
responsible for driving the present accelerated phase of the Universe
expansion. The interplay between Dark Energy and Dark Matter is displayed in
the equations governing the growth of structure in the Universe.
