A general review was made on the recent progress in X-ray spectroscopy. As examples of our special interest the following subjects were chosen: EXAFS and its application to structural analysis, spectroscopic studies of transition-metal compounds and rare-earth compounds, and inter-relationship among the spectra in the X-ray energy region obtained by means of different methods. With the help of electron orbital radiation as an intense continuous X-ray source and improved theoretical treatments the study of X-ray spectra has been more quantitative than qualitative compared with that in ten years ago.
Structure and function of a protein proteinase inhibitor SSI (StreptomycesSubtilisin Inhibitor) are discussed on the basis of the crystal structure analyses of SSI alone and the complex of SSI with subtilisin. A retrospective review of the whole process of the structure analyses is given.