The magnetic, mechanical or chemical properties of nanocrystalline materials strongly differ from the ones of their coarse-grained counterparts. Moreover, significant changes of the phase diagrams were already evidenced for nanostructured alloys. Thermal processing with or without applied pressure controls the microstructure development at the nanometer scale and thus essentially decides upon the final nanomaterial behaviour and properties. A common route for the synthesis of metallic nanomaterials is the devitrification of amorphous precursors obtained via non-equilibrium processing, e.g. by rapid solidification or high-energy ball-milling. Time-resolved in-situ X-ray diffraction experiments may nowadays be performed at high-brilliance synchrotron radiation sources for a variety of temperature-pressure conditions. The temperature-time evolution of the grain-size distribution and microstrain can be monitored in detail at specimen-relevant scales. Together with local information from electron microscopy and chemical analysis, in-situ X-ray experiments offer a complete set of tools for engineering of the microstructure in nanomaterials. The effect of individual processing steps can be distinguished clearly and further tuned. An example is provided, concerning the high-temperature microstructure development in Co-rich soft magnetic nanostructured alloys.