TTP95-20 Three-loop QCD Corrections to \delta\rho, \DELTA r and \Delta\kappa

QCD corrections to electroweak observables are reviewed. Recent results on contributions from the top-bottom doublet of ${\cal O}(\as^2)$ to $\drho$, $\Delta r$ and $\Delta\kappa$ are presented. It is demonstrated that the first three terms in the expansion in $M_Z^2/M_t^2$ provide an excellent approximation to the exact result. Calculational techniques are briefly discussed.