This paper describes the LEP commissioning experience gained up to now on imperfection diagnosis and possible corrections. It summarizes the work done and the results collected by a number of people on beam optics, collective effects and beam‐beam interactions. Concerning optics imperfections, the questions of the strong betatron coupling observed, its possible sources and ways to reduce the consequent undesirable effects are emphasized. Other observations on dispersion, optics irregularities, chromaticity and dynamic aperture are briefly mentioned. Under collective effects, predictions and measurements of transverse impedances and mode‐coupling instability threshold are presented, together with results on the longitudinal impedance and the unpredicted coupled‐bunch oscillations, damped by feedback. Recent measurements of synchro‐betatron resonance effects observed in varying bunch current and vertical tune are also reported. For beam‐beam interactions, the experimental limit on the beam‐beam strength parameter and its dependence on the tunes are shortly discussed. The concluding remarks underline that the understanding of these imperfections and their correction make it possible a continual improvement of the LEP performance.