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 of N = 1 supersymmetric compactifications the one-loop th
reshold contributions are dominant, while the higher loop correction are su
bleading. This is due to the fact that at heavy Kaluza-Klein levels the spe
ctrum as well as the interactions are N = 2 supersymmetric. In particular,
we give two different arguments leading to this result - one is field theor
etic, while the second one utilizes the power of string perturbation techni
ques. To illustrate our discussions we perform explicit two-loop computatio
ns 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. (C) 1999 Elsevier Science B.V. All rights reserve
d.