X-ray microscopy and time-resolved observation using synchrotron radiation are reviewed based on recent reports on in situ observation of cracking and degradation of structural materials. X-ray microscopy can provide three-dimensional images of microstructures and chemical states with a high spatial resolution down to 50 nm. In situ observation using X-ray microscopy successfully revealed crack initiation and propagation in carbon fiber reinforced plastic (CFRP) while a stress was applied to the specimen. X-ray microscopy was applied to ceramic coating or CFRP to perform chemical-state mapping, which is essential for understanding the trigger sites of degradation. In situ observation of time-resolved X-ray diffraction and/or X-ray absorption spectroscopy was carried out with a time resolution down to a few nanosecond by synchronizing an X-ray pulse with a laser pulse. This technique was applied to investigate the mechanism of deformation and fracture or structural phase transitions of metal and alloys after the laser shock.