The ALEPH detector has been built to measure the events created by electron positron collisions in LEP, at first (1989 to 1995) in the energy range of the Z particle (around 91 GeV) and later (1995 to 2000) above the threshold of W pair production ( up to 200 GeV). Typical events are complex, having many particles distributed in jets over the entire detector volume. The event rate at the peak of the Z is below 1 Hz and at least a factor hundred smaller at the highest energies. The ALEPH detector was therefore designed to accumulate, for each event, as much information over as much solid angle as seemed practical.

     This was achieved by a cylindrical arrangement around the beam pipe, with the electron-positron interaction point in the middle. A magnetic field of 1.5 Tesla is created by a superconducting coil, 6.4 m long and 5.3 m in diameter. The iron return yoke is a dodecagonal cylinder with two end-plates that leave holes for a focusing magnet (quadrupole) of the LEP machine. The iron is 1.2 m thick and is subdivided into layers that leave space for the insertion of the layers of streamer tubes, so that the iron yoke is a fully instrumented hadron calorimeter (HCAL), which is read out in 4608 projective towers. Outside the iron, there are two double layers of streamer tube chambers to record the position and angle of muons that have penetrated the iron.
Inside the coil there follows the electron-photon calorimeter (ECAL), designed for the highest possible angular resolution and electron identification. It consists of alternating layers of lead and proportional tubes read out in 73 728 projective towers, each subdivided into three depth zones.
     The central detector for charged particles is a time projection chamber (TPC), 4.4 m long and 3.6 m in diameter. It provides a three dimensional measurement of each track segment. In addition, it provides up to 330 ionisation measurements for a track; this is useful for particle identification. It surrounds the inner track chamber (ITC), an axial-wire drift chamber with inner and outer diameters of 13 cm and 29 cm and a length of 2 m. It provides 8 track coordinates and a trigger signal for charged particles that come from the interaction point.
     Closest to the beam pipe, there is a silicon strip vertex detector. For each track, it measures two pairs of coordinates, 6.3 cm and 11 cm away from the beam axis over a length of 40 cm along the beam line.
     The beam pipe, made out of beryllium, has a diameter of 16 cm. The vacuum inside is about 10^-15atm.