Uncertainties in coupling constant unification

The status of coupling constant unification in the standard model and its supersymmetric extension are discussed. Uncertainties associated with the input coupling constants, mt, threshold corrections at the low and high scales, and possible nonrenormalizable operators are parametrized and estimated. A simple parametrization of a general supersymmetric new particle spectrum is given. It is shown that an effective scale MSUSY can be defined, but for a realistic spectrum it may differ considerably from the typical new particle masses. The implications of the lower (higher) values of αs(MZ) suggested by low-energy (Z-pole) experiments are discussed.