QCD studies at LEP-1

The high hadronic event statistics collected at the Z energy (LEP I) allowed a good understanding of the QCD dynamics. The coupling constant $\alpha_s$ has been measured with several methods giving a global average $\alpha_s(M_Z) = 0.122 \pm 0.004$. The flavour independence of $\alpha_s$ has been tested obtaining $\alpha_s^b/\alpha_s^{udsc} = 0.997 \pm 0.023$. Quark-gluon jet differencies has been observed among which $ _{gluon}/ _{quark} = 1.234 \pm 0.027$. A big role has been plaied by the silicon vertex detectors.