  The status of coupling constant unification -- assuming the validity of the
standard model or of its minimal supersymmetric extension at high energies --
and of relations between various Yukawa couplings (assuming the supersymmetri
extension) which are implied in certain grand-unified theories, are studied in
detail. Theoretical uncertainties in the calculations are emphasized, and
low-energy constraints and predictions are derived. In particular, we find that
bottom-tau unification favors a Higgs boson lighter than 110 GeV. The structure
of the vacuum in the model studied is also discussed. Implications of embedding
supersymmetric models in grand-unified theories are further explored and are
shown to affect the soft supersymmetry breaking mass parameters of the models,
and thus the spectrum at low-energy.
