  Generalized Robertson-Walker (GRW) spaces constitute a quite important family
in Lorentzian geometry, and it is an interesting question to know whether a
Lorentzian manifold can be decomposed in such a way. It is well known that the
existence of a suitable vector field guaranties the local decomposition of the
manifold. In this paper, we give conditions on the curvature which ensure a
global decomposition and apply them to several situations where local
decomposition appears naturally. We also study the uniqueness question,
obtaining that the de Sitter spaces are the only non trivial complete
Lorentzian manifolds with more than one GRW decomposition. Moreover, we show
that the Friedmann Cosmological Models admit an unique GRW decomposition, even
locally.
