The physical meaning and the potentialities of X-ray absorption spectroscopy (XAS) from core states are illustrated in the framework of multiple-scattering theory. By exploiting the mathematical and physical relationships between photoelectron diffraction (PED) and XAS, the energy modulation of the absorption coefficient above the absorption edge is interpreted as a three-dimensional holographic image of the atomic positions of the sample projected onto the energy axis. The theoretical conditions under which this interpretation is possible are specified, making PED and XAS a powerful tool for structural investigations in surface and material science.