  The past ten years of physics with e+e- colliding experiments at LEP and SLAC
have shown the success of these experiments on not only impressively proving
the theoretical predictions of the Standard Model (SM), but also to help
provide stringent bounds on physics beyond the SM. With this experience in
mind, there appear two equally fascinating opportunities for studying
fermion-pair production processes at a future Linear Collider (LC). On the one
hand, performing high precision measurements to the SM, for example, when
running with high luminosity at the Z boson resonance, could be a quick and
feasible enterprise in order to pin down the symmetry breaking mechanism of the
electroweak sector through indirectly determining the masses of a light SM or
MSSM Higgs boson or supersymmetric particles via virtual corrections. On the
other hand, looking for such particles in direct production or other `New
Physics' effects at energies between, for example, roughly 500 and 800 GeV will
naturally be the main motivation to pursue the challenging endeavor of building
and utilizing such a unique facility. These two scenarios for the LC shall be
sketched here, with particular emphasis on the semi-analytical program ZFITTER
for fermion-pair production in comparison with numerical programs like TOPAZ0,
KK2f, and others.
