The importance of nuclear energy in meeting future energy demands has been well-recognised and a variety of nuclear reactor systems have been developed. Inherent characteristics of nuclear technology like neutron economy and neutron irradiation-induced degradation in properties of materials require stringent control of material purity and necessarily limit the choice of candidate materials. Hence safe, reliable and economic operation of nuclear fission reactors, the source of nuclear power at present, requires judicious choice, careful preparation and specialised fabrication procedures for fuels and fuel element structural materials. These aspects of nuclear fuels (uranium, plutonium and their oxides and carbides), fuel element technology and structural materials (aluminium, zircaloy, stainless steel etc.) are discussed with particular reference to research and power reactors in India, e.g. theDhruva research reactor atBarc, Trombay, the pressurised heavy water reactors (Phwr) at Rajasthan and Kalpakkam, and the Fast Breeder Test Reactor (Fbtr) at Kalpakkam. Other reactors like the gas-cooled reactors operating in UK are also mentioned. Because of the limited uranium resources, India has opted for a three-stage nuclear power programme aimed at the ultimate utilization of her abundant thorium resources. The first phase consists of natural uranium dioxide-fuelled, heavy water-moderated and cooledPhwr. The second phase was initiated with the attainment of criticality in theFbtr at Kalpakkam. Fast Breeder Reactors (Fbr) utilize the plutonium and uranium by-products of phase 1. Moreover,Fbr can convert thorium into fissile U-233. They produce more fuel than is consumed — hence, the name breeders. The fuel parameters of some of the operating or proposed fast reactors in the world are compared.Fbtr is unique in the choice of mixed carbides of plutonium and uranium as fuel. Factors affecting the fuel element performance and life in various reactors e.g. hydriding of zircaloys, fuel pellet-cladding interaction etc. inPhwr and void swelling, irradiation creep and helium embrittlement of fuel element structural materials inFbr are discussed along with measures to overcome some of these problems.