Experimental tests of asymptotic freedom

Measurements which probe the energy dependence of αs, the coupling strength of the strong interaction, are reviewed. Jet counting in e+e-annihilation, combining results obtained in the centre of mass energy range from 22 to 133 GeV, provides direct evidence for an asymptotically free coupling, without the need to determine explicit values of αs. Recent results from jet production in ep and in pp collisions, obtained in single experiments spanning large ranges of momentum transfer, Q2, are in good agreement with the running of αs as predicted by QCD. Mass spectra of hadronic decays of τ-leptons are analysed to probe the running αs in the very low energy domain, 0.7 GeV2<Q2<Mτ2. An update of the world summary of measurements of αs(Q2) consistently proves the energy dependence of αs and results in a combined average of αs(MZ0)=0.118±0.006. (orig.)