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.
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