Overview of various in situ / in operando techniques used to characterize functional materials during normal operation, to investigate the processes of materials synthesis and phase transformations, to isolate transient or metastable states. X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, IR, Raman, diffraction techniques, magnetic resonance techniques, electron microscopy, etc. Advantages and limitations. Important considerations when devising a new in situ set up / probe.