The field of bioinorganic chemistry (or bio-coordination chemistry, as it is referred to in this series) has continued to grow in the first two decades of the 21st century and, consequently, the topics covered in the current volume have evolved since the publication of its CCC-II predecessor in 2003. We have collected contributed chapters in this volume to reflect the state of the art in this field. This introductory chapter summarizes the topics covered, which are categorized by either the types of metal ions and their associated cofactors in the bio-coordination complexes, such as heme, nonheme iron, copper, nickel, cobalt, molybdenum and tungsten, or by the functions these bio-coordination complexes exhibit, such as electron transfer, the activation of dioxygen, dinitrogen, or dihydrogen, and the functionalization of carbon-hydrogen bonds.