We derive the universal threshold corrections in heterotic string theo
ry including a continuous Wilson line, Unification of gauge and gravit
ational couplings is shown to be possible even within perturbative str
ing theory, The relative importance of gauge group dependent and indep
endent thresholds on unification is clarified, Equipped with these res
ults we can then attempt an extrapolation to the strongly coupled hete
rotic string - M-theory, We argue that such an extrapolation might be
meaningful because of the holomorphic structure of the gauge coupling
function and the close connection of the threshold corrections to the
anomaly cancellation mechanism. (C) 1997 Elsevier Science B.V.