Leptons and QCD

Three important QCD-related aspects of the $\tau$ and $\mu$ dynamics are reviewed: the determination of the strong coupling from the hadronic tau decay width, leading to the updated value ${\alpha }_s(m_{\tau }^2)=0.331\pm 0.013$; the measurement of $\mathrm{\mid }{V}_{us}\mathrm{\mid }$ through the Cabibbo-suppressed decays of the $\tau$, and the Standard Model prediction for the muon anomalous magnetic moment.