Innovative fuel technologies are necessary for almost all the Generation IV systems discussed in this chapter. Fuels for fast reactor systems will probably contain significant quantities of TRUs, which require remote manufacturing operations. The vast majority of the world’s nuclear fuel experience thus far has been obtained with ceramic fuel pellets composed of uranium dioxide or mixed (U, Pu) oxides. Nitride or carbide fuels are more likely to be suitable for Generation IV systems given their high melting point, density and thermal conductivity. They might be supplied as composite fuels in which a fissile component is combined with a higher conductivity inert matrix. Although a certain amount of experimental data are available for these fuels, far more information will be necessary in order to fully assess their suitability. Fuel microstructure plays a decisive role in in-reactor performance and changes in microstructure (e. g. grain size, porosity distribution, etc.) and chemistry can cause marked changes in behaviour, particularly with the elevated temperatures and higher burn-ups associated with these technologies.