Radiative corrections to $e^{+}e^{-} \to f^{+}f^{-}$

The past ten years of physics with e+e- colliding experiments at LEP and SLAChave shown the success of these experiments on not only impressively provingthe theoretical predictions of the Standard Model (SM), but also to helpprovide stringent bounds on physics beyond the SM. With this experience inmind, there appear two equally fascinating opportunities for studyingfermion-pair production processes at a future Linear Collider (LC). On the onehand, performing high precision measurements to the SM, for example, whenrunning with high luminosity at the Z boson resonance, could be a quick andfeasible enterprise in order to pin down the symmetry breaking mechanism of theelectroweak sector through indirectly determining the masses of a light SM orMSSM Higgs boson or supersymmetric particles via virtual corrections. On theother hand, looking for such particles in direct production or other `NewPhysics' effects at energies between, for example, roughly 500 and 800 GeV willnaturally be the main motivation to pursue the challenging endeavor of buildingand utilizing such a unique facility. These two scenarios for the LC shall besketched here, with particular emphasis on the semi-analytical program ZFITTERfor fermion-pair production in comparison with numerical programs like TOPAZ0,KK2f, and others