Determination of the QCD Coupling αₛ

Theoretical basics and experimental determinations of the coupling parameter of the Strong Interaction, αₛ, are reviewed. The world average value of αₛ, expressed at the energy scale of the rest mass of the Z⁰ boson, is determined from analyses which are based on complete NNLO perturbative QCD. The result is αₛ (MZ) = 0.1184 ± 0.0031. No significant deviations or systematic biases of subsamples of experimental results are found. From the observed energy dependence of αₛ, which is in excellent agreement with the expectations of QCD, the number of colour degrees of freedom can be constrained to Nc = 3.03 ± 0.12.