It is foreseen that the Gran Telescopio Canarias will include an Adaptive Optics (AO) system to provide diffraction-limited images in the near-IR. The preliminary scientific requirements are presented here and are used to determine the basic system specifications. The sky coverage is determined for the cases of natural and laser guide stars. The results of conjugating the deformable mirror to the entrance pupil and to the mean turbulence height are compared. The Gran Telescopio Canarias will employ a segmented primary mirror. This will introduce discontinuous wavefront errors due to possible effects of these errors on the AO performance. The resulting requirements for these errors are compared with those determined by the error budget for long-exposure image quality.