The dynamical justifications for an effective Statistical Mechanics of self gravitating systems (SGS) are formulated, analyzing some among the well known obstacles thought to prevent a rigorous Statisticaltreatment. The most serious problems are due to the formal unboundness of available phase space and the long range nature of gravitational interaction. As to the first point, we argue nevertheless that a hierarchy of timescales exist such that, at any finite time, the volume of the effectively available region of phase space is indeed finite, and that gravitational N-body dynamics satisfies a strong chaos criterion, supporting the assumption of a fast increasingly uniform spreading of orbits over such effectively invariant region, on a constant(N,V,E) surface, i.e., the evolution towards a generalized microcanonical distribution, which is however secularly evolving, though on timescales much larger than those associated to mixing. The second problem entails the well known fact that the ensemble equivalence is broken and implies the non-extensivity of canonical and grand-canonical thermodynamic potentials.This raises the issue about the meaning and the relevance, for these systems, of the procedure of taking the Thermodynamic Limit and invalidates the justification of the use of canonical ensembles. Instead, on the basis of a suitably generalized microcanonical ensemble here proposed, it is possible to define an effective entropy, which turns out to be extensive (despite the non stable nature of the interaction) and derive an orthode for SGS, which in turns gives consistent definitions for other thermodynamic variables. Moreover, a Second Law-like criterion is used to single out the hierarchy of {\sl secular equilibria} describing, for any finite time, the macroscopic behaviour of SGS.