Size effects in nanostructured (nanocrystalline, nanophase or nanocomposite) materials (NMs) and their stability are of great importance for fundamental considerations and modem practice. The size effect peculiarities in NMs are analyzed and the complex influence of grain size and other factors on NM properties is emphasized. New approaches in the development of thermostable NMs are considered with a special attention to the importance of the reproducibility of NM structures and properties.