  In this paper we deal with some problems concerning minimal hypersurfaces in
Carnot-Caratheodory (CC) structures. More precisely we will introduce a general
calibration method in this setting and we will study the Bernstein problem for
entire regular intrinsic minimal graphs in a meaningful and simpler class of CC
spaces, i.e. the Heisenberg group H^n. In particular we will positively answer
to the Bernstein problem in the case n=1 and we will provide counterexamples
when n>=5.
