  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.
