Abstract：Nanocrystalline – and nanostructured materials offer particular promise for new and potentially very useful products, since they can have very different - and often superior properties that crucially depend on the atomistic details of interior or exterior interfaces. As nanocrystalline materials are structures far away from thermodynamic equilibrium and since they have short transport pathways, fast diffusion and rapid transformation kinetics often lead to coarsening and to the deterioration of the microstructure and the associated properties. Thus, ensuring the stability of the nanoscale structures, e.g. by utilizing a composite approach, is a key issue. This approach, however, imposes additional constraints on processing-related issues as well as on the stability of phases and phase mixtures and on phase transformations within the nanoscale structural units due to size confinement and due to the presence of heterophase interfaces with excess free energy contributions. While the entire field of nanocrystalline composite materials is far too vast to be addressed in a single overview, recent developments concerning advanced synthesis options and concerning phase stability and phase transformation issues of nanocrystalline composite materials as followed at the Institute of Materials Physics of the University of Muenster will be highlighted. These issues are to be considered in general for systems that are characterized by a large specific interface area.