  Hadronic $\tau$ decays provide a clean laboratory for the precise study of
quantum chromodynamics (QCD). Observables based on the spectral functions of
hadronic $\tau$ decays can be related to QCD quark-level calculations to
determine fundamental quantities like the strong coupling constant, quark and
gluon condensates. Using the ALEPH spectral functions and branching ratios,
complemented by some other available measurements, and a revisited analysis of
the theoretical framework, the value $\asm = 0.345 \pm 0.004_{\rm exp} \pm
0.009_{\rm th}$ is obtained. Taken together with the determination of \asZ from
the global electroweak fit, this result leads to the most accurate test of
asymptotic freedom: the value of the logarithmic slope of $\alpha_s^{-1}(s)$ is
found to agree with QCD at a precision of 4%. The value of \asZ obtained from
$\tau$ decays is $\asZ = 0.1215 \pm 0.0004_{\rm exp} \pm 0.0010_{\rm th} \pm
0.0005_{\rm evol}
  = 0.1215 \pm 0.0012$.
