Extraction of the strong coupling constant and strange quark mass from   semileptonic tau decays
  In this lecture I present a pedagogical introduction to the low-energy phenomenology of light flavors. The renormalization scheme freedom in defining QCD parameters is discussed. It is shown in some details how one can extract an accurate numerical value for the strong coupling constant from the tau-lepton decay rate into hadrons. As a related topic I discuss some peculiarities of definition of the quark mass in theories with confinement and describe the strange quark mass determination from data on tau-lepton decays employing contour resummation which is a modern technique of the precision analysis in perturbative QCD. 
A.A. Pivovarov