  We discuss higher loop corrections to gauge coupling renormalization in the
context of gauge coupling unification via Kaluza-Klein thresholds. We show that
in the case N=1 supersymmetric compactifications the one-loop threshold
contributions are dominant, while the higher loop correction are subleading.
This is due to the fact that at heavy Kaluza-Klein levels the spectrum as well
as the interactions are N=2 supersymmetric. In particular, we give two
different arguments leading to this result - one is field theoretic, while the
second one utilizes the power of string perturbation techniques. To illustrate
our discussions we perform explicit two-loop computations of various
corrections to gauge couplings within this framework. We also remark on
phenomenological applications of our discussions in the context of TeV-scale
brane world.
