  GUT scale threshold corrections in minimal SU(5) supergravity grand
unification are discussed. It is shown that predictions may be made despite
uncertainties associated with the high energy scale. A bound relating the
strong coupling constant to the mass scales associated with proton decay and
supersymmetry is derived, and a sensitive probe of the underlying theory is
outlined. In particular, low energy measurements can in principle determine the
presence of Planck scale ($ 1 / {{\rm M}_{\rm Pl}} $) terms.
