X-ray absorption spectroscopy (XAS) is a key tool for studying bioinorganic systems, shedding light on the interactions between metals and biomolecules. This book discusses its principles, experimental techniques and applications, particularly in metalloproteins and catalytic systems. XAS has made it possible to elucidate essential biological mechanisms and to better understand the role of metals in biology. Research into these systems is crucial to biomedicine, particularly for the development of targeted therapies and the prevention of diseases linked to metal imbalances. The study of metal toxicity also underlines the importance of public health and environmental protection. An interdisciplinary approach involving chemists, biologists and environmental specialists is needed to meet these challenges. Thanks to technological advances, XAS will continue to enrich our understanding of vital processes and improve our living conditions.