The advent of Extremely Large Telescopes impose the existence of wavefront correctors to achieve diffraction limited images. The light collection area is more evidently affected by atmospheric turbulences and image distortion is more relevant than in smaller telescopes. AO system where mostly used to improve the performances while now turn to be a real need. Though the existence of a wavefront reconstructor is mandatory for the next generation of telescopes, the requirements imposed for such large apertures are to be examined and the new challenges must be faced.