The Strong Coupling Constant in Grand Unified Theories

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.