OPERA is an hybrid detector: since the emulsion sheets have no time resolution, electronics detectors are needed to correlate the occurrence of neutrino events to that target element where the interaction occurs and where scanning must start. Each wall built of 3328 bricks is equipped of two plan of electronic trackers (plastic scintillators read out by wavelength-shifting fibres). Each target supermodule consists of 31 walls. A long-baseline neutrino beam pointed to the Gran Sasso laboratory is under construction at CERN. The SPS primary proton beam at 400 GeV is focused onto a graphite target, producing secondary mesons. Neutrinos are produced in a 900 m length vacuum tunnel by the decay in flight of high momenta p+ and K+ selected and focused towards the Gran Sasso Laboratory.?