The search for fundamental interaction laws continuously stimulates the accelerator community to increase the energy of accelerators. The present need for high-accuracy measurements in already explored energy regions requires a new generation of colliders, the e+ e− factories. These should work at the energies of hadronic resonances and produce a very high rate of events with a luminosity increased by at least two orders of magnitude with respect to the present values. Such high luminosity poses interesting new problems to both the detector and accelerator physicists.