The progress already made in studies of the stability of shock waves and some tasks for the future are reviewed. The following aspects of the problem are discussed: 1) stability of shock waves as hydrodynamic discontinuities irrespective of the events which occur in the relaxation zone of a wave (this aspect includes also the problem of stability of a shock wave sustained by a piston and anomalies in the reaction of a wave to external perturbation, including reflection and refraction of perturbations); 2) stability of flow in the relaxation zone (structural stability) of a shock wave. The stress is on the theoretical side of the problem. However, potential practical realization of the shock-wave instability criteria are also considered. Schemes of decay of unstable shock-wave discontinuities are discussed.