Determination of the QCD coupling αs

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 FI om analyses which are based on complete next-to-next-to-leading-order (NNLO) perturbative quantum chromodynamics (QCD). The result is alpha(s)(M-Z0) = 0.1184 +/- 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 +/- 0.12.