Studies of QCD at Centre-of-Mass energies between 91 and 209 GeV

Observables measured with the ALEPH detector at LEP in e^+e^- annihilations into hadronic final states are analysed in the framework of QCD. The measurements comprise event-shape variables and inclusive charged particle spectra. The energy evolution of derived quantities is studied in detail and compared to analytic QCD predictions. The mean charged particle multiplicity, the charged particle momentum spectrum and its peak position are compared to predictions of the modified-leading-logarithmic approximation. The strong coupling constant alpha_s is determined from a fit of the QCD prediction to distributions of six event-shape variables at eight centre-of-mass energies.