  Determinations of the strong coupling strength, $\alpha_s$, are summarised
and a new world average value of $\alpha_s (M_Z)$ is determined, using a new
method of pre-averaging results within classes of measurements like hadronic
$\tau$ decays, deep inelastic scattering processes, lattice calculations,
electron-positron annihilation processes and electro-weak precision fits. The
overall result is $$\alpha_s (M_Z) = 0.1184 \pm 0.0007,$$ unchanged from the
value obtained in 2009. This presentation is an excerpt from the QCD review
section of the 2012 Review of Particle Physics (RPP) of the Particle Data
Group.
