Determination of the QCD coupling α s

Theoretical basics and experimental determinations of the coupling parameter of the strong interaction, α s , are reviewed. The world average value of α s , expressed at the energy scale of the rest mass of the Z 0 boson, is determined from analyses which are based on complete next-to-next-to-leading-order (NNLO) perturbative quantum chromodynamics (QCD). The result is α s ( M Z 0 ) = 0.1184±0.0031. No significant deviations or systematic biases of subsamples of experimental results are found. From the observed energy dependence of α 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.