  We present a method to control the regularization scheme dependence in the
running of couplings in Kaluza-Klein theories. Specifically we consider the
scalar theory in five dimensions, assuming that one dimension is compactified
and we study various regularization schemes in order to analyze concretely the
regularization scheme dependence of the Kaluza-Klein threshold effects. We find
that in one-loop order, although the $\beta$-functions are different for the
different schemes, the net difference in the running of the coupling among the
different schemes is very small for the entire range of energies. Our results
have been extended to include more than one radii, and the gauge coupling
unification is re-examined. Strings are also used as a regulator. We obtain a
particular regularization of effective field theory which can accurately
describe the string Kaluza-Klein threshold effects.
