This book explores in depth the crucial role of metalloproteins in various biological processes, highlighting their importance in sustaining life. Focusing on the principles and applications of XANES spectroscopy, it offers a detailed overview of this powerful technique for studying metalloproteins. Through concrete examples and detailed explanations, the book examines how XANES spectroscopy enables precise quantification of metalloprotein concentrations in biological samples. By better understanding the molecular mechanisms involved in reactions catalyzed by metalloproteins, researchers are able to open up new perspectives in biological research and biotechnology, offering promising opportunities for future applications in fields such as pharmacology, biochemistry and structural biology.