We present a theoretical study of the basic interactions occurring at supported hetereogenous Ziegler-Natty catalytic systems. We first investigated the interaction between prototypes of each class of industrially relevant internal donors (1,3-diethers, alkoxysilanes, phthalates, succinates) and the MgCl2 support. Our analysis indicates that donors can be separated into two classes. 1,3-Diethers and alkoxysilanes belong to the former because they have a short spacer between the coordinating O atoms and,coordinate preferentially to the same Mg atom of the (110) lateral cut. Conversely, phthalates and succinates belong to the latter class because they have a longer spacer between the coordinating O atoms and thus can adopt a variety of coordination modes. Indeed, they can coordinate to both the (100) and (110) lateral cuts. In the last part of this manuscript we report on the stereo- and regioselective behavior of possible active Ti species with and without two succinate molecules coordinated in the proximity of the Ti atom. We show that the two succinate molecules confer a remarkable stereoselectivity in both primary and secondary propene insertions. This model very simply rationalizes the effect of the donors, and it is consistent with the models so far developed to rationalize the stereoselectivity of metallocene and octahedral nonmetallocene catalysts.