Determination of the QCD Coupling $\alpha_s$

Theoretical basics and experimental determinations of the coupling parameter of the Strong Interaction, $\alpha_s$, are reviewed. The world average value of $\alpha_s$, expressed at the energy scale of the rest mass of the $Z^0$ boson, is determined from analyses which are based on complete NNLO perturbative QCD. The result is $\alpha_s (M_Z) = 0.1184 \pm 0.0031$. No significant deviations or systematic biases of subsamples of experimental results are found. From the observed energy dependence of $\alpha_s$, which is in excellent agreement with the expectations of QCD, the number of colour degrees of freedom can be constrained to $N_c = 3.03 \pm 0.12$.