The 10m Gran Telescopio Canarias (GTC) is currently being installed in the Observatorio del Roque de los Muchachos (ORM) on the island of La Palma. An adaptive optics (AO) system will be installed at one of the Nasmyth foci of the telescope within a year of the telescope being commissioned. The preliminary design of the adaptive optics system is presented here. The system will initially be operated in single-conjugate mode using a natural guide star, but provisions are made for upgrade to dual-conjugate operation and the use of laser guide stars. The main system requirements and the optical and mechanical design solutions are outlined here. It is planned to employ a piezo-stack deformable mirror having approximately 350 actuators and a Shack-Hartmann wavefront sensor. The tip-tilt correction will be provided by the secondary mirror of the GTC which is a lightweighted Beryllium mirror with a drive system capable of fast tip-tilt and chopping. In preparation for dual-conjugate operation we have studied the optimal altitude of the second deformable mirror (the first will be conjugate to the telescope pupil) using numerical simulations and measurements of turbulence obtained at the ORM. We have used the GSC II catalogue to determine sky-coverage for multi-natural guide star wavefront sensing, as required for dual-conjugate operation. In addition we have investigated a novel approach to multi-object wavefront sensing based on curvature sensing.