The centenary of the discovery of X-rays is the occasion to summarize the history of X-ray absorption spectroscopy (XAS) and its development on synchrotron radiation sources, to give the underlying principles and to illustrate the possible uses of this spectroscopy by three examples: a) rile structural characterization of disordered spin transition coordination compounds by EXAFS spectroscopy; b) the ligand binding to vitamin B-12 by XANES spectroscopy; and c) the chemical analysis of sulfur compounds included in fly-ashes by low-energy XANES spectroscopy.