Achieving high burn-ups of the fuel used in fast-neutron reactors is essential for the economy of the breeder system. The mixed-oxide fuel (UO$_{2}$, PuO$_{2}$), which is generally used, enclosed in metallic pins assembled together inside an hexagonal wrapper, suffers high irradiation damage in a very hostile environment (high temperature, severe temperature gradients, corrosive atmosphere) for long periods of time. We review the physical phenomena that are in action during the lifetime of a fuel assembly and evaluate the predictable performances of mixed oxide breeder fuel. Five factors are specially described: fuel swelling, fuel cladding interaction, clad swelling, fuel behaviour under transients and subassembly behaviour in a reactor core.