[en] In this thesis, precision tests of the electroweak theory with the DELPHI detector at LEP are presented. Two main areas of experimental investigation have been pursed. In the first, the Z^o resonance parameters were determined through precise measurements of the cross section for Z^o production and decay into hadronic and leptonic final states at several collision energies close to the peak of the resonance. Precise measurements of the resonance parameter (the mass, the width, and the peak cross section) provide interesting constraints on the electroweak theory. In particular, the width and the peak cross section depend on the number of neutrino species in nature. The LEP data showed that the number of neutrinos is three. In the second main area of investigation the longitudinal polarisation of tau leptons produced in Z^o decays was determined. The polarisation and its dependence on the production polar angle was deduced from an analysis of the kinematics of the tau decay products. About 40x10³ tau pair events from Z^o decays were analysed. Assuming electron-tau universality the results of the measurement can be expressed in terms of the effective weak mixing angle. 74 refs