The book provides a synthetic and exhaustive view of how proteins have evolved their structure to accommodate a variety of metal ions in order to accomplish the most demanding chemical reactions that sustain life. This book may be the subject of a monographic course on its own or may complement bioinorganic chemistry textbooks to obtain a comprehensive view of the structural aspects of this subject. It is directed at advanced undergraduate and graduate students, researchers, instructors, and professors teaching in protein science, biochemistry, cell biology. This is the first textbook focused on 3D-structures of metalloproteins. It is also a tool for a monographic course on one or more biogeochemical cycles of metal ions. It provides a comprehensive overview of the structural chemistry of metal ions in proteins.