Nanocrystalline materials reveal promising mechanical properties at room temperature, but often quickly degrade due to spontaneous grain growth. In the first part of this overview it is discussed with the help of Cu(Cr) alloys how nanocrystalline metals with improved thermal stability can be designed. In situ and analytical TEM/STEM studies reveal the formation of a nanocomposite microstructure upon annealing of the initial supersaturated Cu(Cr) alloys, which in turn suppresses grain growth and conserves the exceptional strength. The second part provides strategies for testing mechanical properties of interfaces inside electron microscopes. In many cases the dimensions of the microstructure components are too small to extract information from macroscopic mechanical tests. Thus, quantitative mechanical tests must be adapted to the appropriate length scale. Challenges of in situ nanomechanical testing and recent results obtained on specific interfaces are discussed.