The 2009 world average of αs

Measurements of alpha(s), the coupling strength of the Strong Interaction between quarks and gluons, are summarised and an updated value of the world average of alpha(s)(M(Z)0) is derived. Special emphasis is laid on the most recent determinations of alpha(s). These are obtained from tau-decays, from global fits of electroweak precision data and from measurements of the proton structure function F-2, which are based on perturbative QCD calculations up to O(alpha(4)(s)); from hadronic event shapes and jet production in e(+)e(-) annihilation, based on O(alpha(3)(s)) QCD; from jet production in deep inelastic scattering and from Gamma decays, based on O(alpha(2)(s)) QCD; and from heavy quarkonia based on unquenched QCD lattice calculations. A pragmatic method is chosen to obtain the world average and an estimate of its overall uncertainty, resulting in alpha(s)(M(Z)0) = 0.1184 +/- 0.0007. The measured values of alpha(s)(Q(2)), covering energy scales from Q = M-tau = 1.78 GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept of Asymptotic Freedom.