LEP results on $\alpha_{s}(M_{Z})$ from $R_{had}$ and event shape variables

We present here the LEP results on the measurement of the QCD coupling constant, a,, using two complementary analyses. Firstly, from the fits within the framework of the Standard Model, we determine a,(Mz) = 0.127 ± 0.006. Secondly, from the study of global event shape variables, we obtain a,(Mz) = 0.123 ± 0.006. Both measurements agree within the errors. The latter method of determining a; is dominated by theoretical uncertainties.