The technology and components required to implement discrete adaptive optics systems capable of compensating wavefront errors caused by atmospheric turbulence in ground-based astronomical telescopes are reviewed. Characteristics of the major types of deformable mirrors, wavefront sensors and wavefront reconstructors are described. The effects of device limitations such as the size of the compensation subapertures and the signal to noise ratio of the wavefront sensor detector on the overall performance of adaptive optics systems are discussed. This review indicates that the technology exists to enable conventional adaptive optics systems to perform close to their inherent performance limits, the major impediment being the high cost of the components required. However, a larger problem exists in that the usefulness of adaptive optics for ground-based astronomy is severely limited by external factors such as the small size of the isoplanatic patch and the small photon flux available from most astronomical objects. The conclusion is that new system concepts are needed to overcome these external limitations and to make adaptive optics a useful technique for ground-based astronomy. Among the new approaches that have already been proposed are laser guide stars and multiple wavefront correctors.