  We derive expressions for the cross section of the multiparton interactions
based on the analysis of the relevant Feynman diagrams. We express the cross
sections through the double (triple, ...) generalized parton distributions
(GPDs). In the mean field approximation for the double GPDs the answer is
expressed through the integral over two gluon form factor which was measured in
the exclusive DIS vector meson production.We explain under what conditions the
derived expressions correspond to an intuitive picture of hard interactions in
the impact parameter representation. The mean field approximation in which
correlations of the partons are neglected fail to explain the data, while pQCD
induced correlation enhance large $p_\perp$ and $ 0.001 < x < 0.1$ typically
enhance the cross section by a factor of 1.5 -- 2 explaining the current data.
We argue that in the small x kinematics ($10^{-4} \le x \le 10^{-3}$) where
effects of perturbative correlations diminish, the nonperturbative mechanism
kicks in and generates positive correlations comparable in magnitude with the
perturbative ones. We explain how our technique can be used for calculations of
MPI in the proton - nucleus scattering. The interplay of hard interactions and
underlying event is discussed, as well as different geometric pictures for each
of MPI mechanisms-pQCD, nonperturbative correlations and mean field.
Predictions for value of \effs for various processes and a wide range of
kinematics are given. We show that together different MPI mechanisms give good
description of experimental data, both at Tvatron, and LHC, including the
central kinematics studied by ATLAS and CMS detectors, and forward (heavy
flavors) kinematics studied by LHCb.
