The 2009 Wolrd Average of $α_s (M_Z)$

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 $\alpha_s (M_Z)$ is derived. Building up on previous reviews, 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 $O(\alpha_s^4)$; from hadronic event shapes and jet production in $\epem$ annihilation, based on $O(\alpha_s^3) $ QCD; from jet production in deep inelastic scattering and from $\Upsilon$ decays, based on $O(\alpha_s^2) $ QCD; and from heavy quarkonia based on unquenched QCD lattice calculations. Applying pragmatic methods to deal with possibly underestimated errors and/or unknown correlations, the world average value of $\alpha_s (M_Z)$ results in $\alpha_s (M_Z) = 0.1184 \pm 0.0007$. The measured values of $\alpha_s (Q)$, covering energy scales from $Q \equiv \mtau = 1.78$ GeV to 209 GeV, exactly follow the energy dependence predicted by QCD and therefore significantly test the concept af Asymptotic Freedom.