Many-body final states in X-ray absorption near-edge structure (XANES) using synchrotron radiation are a unique tool for probing quantum many-body initial states in complex electronic correlated systems where single-particle descriptions of electronic states in a periodic potential break down. Multi-electron excitations were first observed in the 1960s in X-ray absorption spectra of atoms and subsequently in molecules and complex 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.