QCD Corrections to Electroweak Precision Observables in SUSY Theories

The two-loop QCD corrections to the $\rho$ parameter are derived in the Minimal Supersymmetric Standard Model. They turn out to be sizable and modify the one-loop result by up to 30%. Furthermore exact results for the gluonic corrections to $\mathrm{\Delta }r$ are presented and compared with the leading contribution entering via the $\rho$ parameter.