  We prove a subelliptic estimate for systems of complex vector fields under
some assumptions that generalize the essential pseudoconcavity for $CR$
manifolds and H\"ormander's bracket condition for real vector fields.
Applications are given to prove the hypoellipticity of first order systems and
second order partial differential operators. Finally we describe a class of
compact homogeneous CR manifolds for which the distribution of $(0,1)$ vector
fields satisfies a subelliptic estimate. v2: minor revision, to appear in Ann.
Inst. Fourier
