Abstract: Covalency in lanthanide and actinide compounds, i.e., role of f- and d-orbitals in chemical bonding, continues to be at the forefront of research in the chemical science. X-ray absorption spectroscopy has emerged as an effective and direct method to quantitively evaluate the covalency in chemical bonding. In this review paper, the theoretical background was briefly introduced for X-ray absorption spectroscopy and density functional theory. The application of X-ray absorption spectroscopy and density functional theory in the investigation of the covalency in lanthanide and actinide compounds was summarized. Finally, the prospect was discussed.