  The x-ray absorption near edge structure (XANES), developed in these last 40
years using synchrotron radiation, is a unique tool probing electronic
correlations in complex systems via quantum many body final state effects.
Multi electron excitations have been observed first in the sixties in x-ray
absorption spectra of atoms and later in molecules and solids. The applications
of XANES many body final states to probe unique features of electronic
correlation in heavy fermions, mixed valence systems, mixed valence oxides and
high temperature superconductors are discussed.
