Over the past decade the most successful method of fuelling tokamaks has been simply to inject the gas (deuterium or hydrogen) into the vacuum chamber via a fast acting, electronically controllable valve. This valve gives either a pre-programmed flow of gas, or is controlled by a servosystem that tracks the plasma density and maintains it at a pre-set level below the critical density at which the plasma current will disrupt; the density limit. To date this method of fuelling has worked adequately, but there are drawbacks that must be avoided as experiments approach fusion conditions. A recent technique developed for fuelling a reacting plasma is that of injecting a high speed pellet of frozen fuel (a mixture of tritium and deuterium) directly into the plasma core. This method offers several advantages over that of gas puffing: much higher; it can be used to control pure fuel of the pellet dilutes the the efficiency in fuel delivery is and peak the density profile; the impurities in the plasma and the impurity influx from the material surfaces interacting with the plasma may be reduced.