  The four-dimensional superstring solutions define at low energy effective
supergravity theories. A class of them extends successfully the validity of the
standard model up to the string scale (${\cal O}(10^{17})~TeV$). We stress the
importance of string corrections which are relevant for low energy (${\cal
O}(1)~TeV$) predictions of gauge and Yukawa couplings as well as the spectrum
of the supersymmetric particles. A class of exact string solutions are also
presented, providing non trivial space-time backgrounds, from which we can draw
some lessons concerning the regions of space-time where the notion of the
effective field theory prescription make sense. We show that the string
gravitational phenomena may induce during the cosmological evolution,
transitions from one effective field theory prescription to a different one
where the geometrical and topological data, as well as the relevant observable
states are drastically different. Talk presented by C. Kounnas in the XXIX
Moriond Meeting, M\'eribel, France.
