  The proposed $e^+e^-$-collider FCC-ee aims at an unprecedented accuracy for
$e^+e^-$ collisions into fermion pairs at the $Z$ peak, based on about
$10^{13}$ events. The S-matrix approach to the $Z$ boson line shape allows the
model-independent quantitative description of the reaction $e^+e^- \to {\bar
f}f$ around the $Z$ peak in terms of few parameters, among them the mass $M_Z$
and width $\Gamma_Z$ of the $Z$-boson. While weak and strong corrections remain
"black", a careful theoretical description of the photonic interactions is
mandatory. I introduce the method and describe applications and the analysis
tool SMATASY/ZFITTER.
