The different aspects of nanostructured material (NM) stability such as thermal and chemical stability as well as NM behavior under deformation and radiation are characterized and analyzed in detail. Grain growth, phase transitions (including spinodal decomposition), homogenization diffusion processes, relaxation of residual stresses, and behavior of grain boundary and triple junction segregations are discussed in context of the change of nanostructure and properties. Special interest is given to the availability of NMs with ultra-fine grain size and their behavior during annealing as soon as to the possibility of development of nanostructures with high thermal stability. Some unsolved problems are emphasized. (C) 2003 Kluwer Academic Publishers.