The 1990 Cargese Summer Institute on ZO-Physics was organized by the Univer- site Pierre et Marie Curie, Paris (M. Levy and J.-L. Basdevant), CERN (M. Jacob), the Universite Catholique de Louvain (D. Speiser and J. Weyers), and the Katholieke Universiteit te Leuven (R. Gastmans), which, since 1975, have joined their efforts and worked in common. It was the ninth Summer Institute on High Energy Physics orga- nized jointly at Cargese by these three universities. Because of the start-up of LEP in the summer of 1989, we broke with our tradition of having our Summer Institutes in the odd years. Indeed, it seemed to us that the many new data from LEP had to be presented in detail as soon as possible in order to prepare the young researchers in particle physics better for the experimental results with which they will be confronted in the coming years. The main theme of the school was therefore ZO-physics, with particular emphasis on the way the experiments at LEP are analyzed. We had one lecturer from each LEP experiment: they agreed among each other to present different topics in e+e- physics. Nevertheless, they made sure that all the major topics were discussed and that the results could be critically compared.