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, ${\mathit{m}}_{\mathit{t}}$, 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 ${\mathit{M}}_{\mathrm{SUSY}}$ 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 ${\mathrm{\alpha }}_{\mathit{s}}$(${\mathit{M}}_{\mathit{Z}}$) suggested by low-energy (Z-pole) experiments are discussed.