After being member of the Scientific Policy Committee at CERN, Schopper was elected director general and served from 1981 to 1988. His first task was to unite the two CERN laboratories existing at that time under two Directors General. The Large Electron–Positron Collider (LEP) was also proposed and constructed under his leadership. This facility allowed the verification of the standard model of particle physics, namely that it is a renormalizable field theory, leading to the award of the Nobel Prize to the theoreticians Veltman and t'Hooft. Furthermore, it enabled the precise determination of fundamental parameters of the electroweak force, such as the W± and Z masses, and proved the existence of three neutrino families. Thus, this particle accelerator transformed high energy physics into a field of precision measurements and provided estimates to the mass of the top quark, Higgs boson and other supersymmetric and hypothetical particles. LEP was approved under the condition of a reduced and constant budget with the consequence that some unique activities at CERN (e.g. ISR) had to be abandoned. Schopper was obliged to introduce a new way of international collaboration for the four LEP experiments since CERN could not provide funds for them. The experiments became rather independent activities organised in a rather democratic way bringing together hundreds of scientists from many universities and national organisations. The LEP experiments became a model for the later LHC experiments, shaping the way this organization works today. He contributed to the globalisation of research at CERN and was also responsible for the return of Spain and Portugal in the CERN's member states.