Tests of Quantum Chromo Dynamics at e^+e^- Colliders

The current status of tests of the theory of strong interactions, Quantum Chromo Dynamics (QCD), with data from hadron production in e^+e^- annihilation experiments is reviewed. The LEP experiments ALEPH, DELPHI, L3 and OPAL have published many analyses with data recorded on the Z^0 resonance at sqrt(s)=91.2 GeV and above up to sqrt(s)>200 GeV. There are also results from SLD at sqrt(s)=91.2 GeV and from reanalysis of data recorded by the JADE experiment at 14<sqrt(s)<44 GeV. The results of studies of jet and event shape observables, of particle production and of quark gluon jet differences are compared with predictions by perturbative QCD calculations. Determinations of the strong coupling constant alpha_S(M_Z) from jet and event shape observables, scaling violation and fragmentation functions, inclusive observables from Z^0 decays, hadronic tau decays and hadron production in low energy e^+e^- annihilation are discussed. Updates of the measurements are performed where new data or improved calculations have become available. Finally, investigations of the gauge structure of QCD are summarised.