The feasibility of re-fuelling a fusion reactor by injecting pellets of frozen hydrogen isotopes is reviewed. First a general look is taken of the dominant energy fluxes received by the pellet, the re-fuelling rate required and the relation between pellet size, injection speed and frequency. Current available theories of pellet ablation are then discussed. For a given penetration depth inside the reactor, the necessary pellet injection speed is examined in terms of the ablation theory adopted and the temperature and density profiles of the reactor plasma. The interaction between the injected pellet and the background plasma is described with reference to some of the avaialbe tokamak transport codes; its relation to the ignition requirement is mentioned. Various types of pellet sources and different approaches to injection are described and assessed. Past experimental efforts on pellet ablation are summarized and compared with theories. Subjects requiring further investigation are pointed out.