  In this paper, we establish a generalized maximum principle for
pseudo-Hermitian manifolds. As corollaries, Omori-Yau type maximum principles
for pseudo-Hermitian manifolds are deduced. Moreover, we prove that the
stochastic completeness for the heat semigroup generated by the sub-Laplacian
is equivalent to the validity of a weak form of the generalized maximum
principles. Finally, we give some applications of these generalized maximum
principles.
