The large fraction of atoms situated at internal interfaces modifies the properties and behavior of nanostructured or nanocrystalline materials with respect to their conventional, coarse-grained counterparts. Such modifications can extend materials properties to ranges otherwise inaccessible to materials with conventional microstructures. One very prominent example is given by the mechanical properties. The size of coherently scattering regions starts to affect the mechanical properties of a material even when the characteristic length scale is on the micron scale. The behavior of the materials, however, can change in a noncontinuous manner at very small sizes of the coherently scattering regions. Therefore, attention is devoted to address the underlying fundamental issues related to interface structure, interface properties, and defect interactions in nanostructured materials. The thermodynamic properties of a given material address the impact of the length scale of the internal “microstructure” of this material and its internal interfaces. These properties are immediately related to the maps of materials science (i.e., the constitutional phase diagrams that govern stability, composition selection, processing conditions, and performance of the materials). It is well known that the characteristic temperatures for phase transformations - such as melting - change when the characteristic size of a system is reduced to the nanoscale. However, is it “just” the size that controls the shift of the transformation temperature? Is just the characteristic temperature that is affected, or also, more generally, the so-called “thermodynamic potentials”? Is the way thermodynamics is described for multi-component materials still valid for nanostructured materials? In this talk, examples concerning selected properties highlighting the role and importance of internal interfaces and their atomic structure and related properties will be given.