The unification of gauge couplings suggests that there is an underlying
(supersymmetric) unification of the strong, electromagnetic and weak
interactions. The prediction of the unification scale may be the first
quantitative indication that this unification may extend to unification with
gravity. We make a precise determination of these predictions for a class of
models which extend the multiplet structure of the Minimal Supersymmetric
Standard Model to include the heavy states expected in many Grand Unified
and/or superstring theories. We show that there is a strong cancellation
between the 2-loop and threshold effects. As a result the net effect is smaller
than previously thought, giving a small increase in both the unification scale
and the value of the strong coupling at low energies.Comment: 20 pages, Latex, 5 Postscipt figures; 2 references adde
