Threshold Corrections in Grand Unified Theories

We perfrom a state-of-the-art gauge coupling unification analysis for two simple supersymmetric GUT model. We find that in Minimal Supersymmetric SU(5) three-loop effects raise the mass of the colored Higgs triplet by one order of magnitude, attenuating presumed tension with the proton decay bound. In a second part of the thesis, we calculate the yet missing two-loop GUT threshold corrections in the simplest GUT model, the Georgi-Glashow SU(5) and give many technical details of the calculation.