Reduction of couplings in supersymmetric GUTs is achieved by searching
for renormalization group invariant (RGI) relations among couplings w
hich hold beyond the unification scale. Finiteness is due to the fact
that there exist RGI relations among couplings that guarantee the vani
shing of the beta-functions of a N = 1 supersymmetric GUT even to all
orders in perturbation theory. Of particular interest are the relation
s among gauge and Yukawa couplings which lead to very interesting pred
ictions of the top quark mass.