[en] The first experimental evidence in favour of a unified description of the weak and electromagnetic interactions was obtained in 1973 with the observation of neutrino interactions resulting in final states which could only be explained by assuming that the interaction was mediated by the exchange of a massive, electrically neutral virtual particle. Within the framework of the Standard Model, these observations provided a determination of the weal mixing angle,θ_w, which, despite its large experimental uncertainty, allowed the first quantitative prediction for the mass values of the weak bosons, W ± and Z. The numerical values so obtained ranged from 60 to 80 GeV for the W mass, and from 75 to 92 GeV for the Z mass, too large to be accessible by any accelerator in operation at that time. (Author)