The term nuclear materials refers to a very wide range of materials that are encountered in the nuclear fuel cycle. Such materials usually contain radioactive elements such as actinides, fission and activation products, and they are generally heterogeneous, chemically diverse and in some cases amorphous (glasses) or have a crystalline structure affected by radioactive decay. The use of XAS on highly radioactive samples such as spent nuclear fuels and nuclear glasses is still in its infancy, as it has only been recently that the combination of sample preparation in hot laboratories and the availability of beamlines dedicated to radioactive materials has allowed researchers to work on such samples. Through various research results on nuclear fuels, cladding materials, transmutation targets and nuclear glasses, this chapter illustrates how XAS provides crucial data for the fundamental comprehension of these materials.