  The prediction of the strong coupling constant in grand unified theories is
reviewed, first in the standard model, then in the supersymmetric version.
Various corrections are considered. The predictions in both
supergravity-induced and gauge-mediated supersymmetry breaking models are
discussed. In the region of parameter space without large fine tuning the
strong coupling is predicted to be alpha_s(M_Z) > 0.13. Imposing
alpha_s(M_Z)=0.118, we require a unification scale threshold correction of
typically -2%, which is accommodated by some GUT models but in conflict with
others.
