The biological activation of dioxygen is a key reaction in biological systems. Enzymes involved in direct oxygen activation are oxidases and oxygenases. Multi-copper oxidases are an important class of oxidases reducing dioxygen in a four-electron reduction to water with concomitant one-electron oxidation of the reducing substrate. The progress in the characterization and understanding of the structure and function of these enzymes has advanced so tremendously over the last ten years that the publication of a book documenting these achievements has been overdue.Especially the recent discovery of a key role of the FET3 protein of Saccharomyces cerevisae, a multi-copper oxidase, in iron metabolism of this eukaryote has underpinned the function of the plasma multi-copper oxidase ceruloplasmin in vetebrate iron transport. The lately determined x-ray structure of human ceruloplasmin confirms its close structural relatedness to the plant multi-copper oxidases ascorbate oxidase and laccase and due to strong amino-acid sequence similarities has allowed to construct a useful model of the more distantly related blood-clotting factor VIII.This book contains review articles from experts in the field, dealing with modern spectroscopy, enzyme kinetics, bioinorganic chemistry, x-ray crystallography, electron transfer reactions, molecular biology, medical aspects and potential industrial applications of the three main members of multi-copper oxidases, i.e., laccase, ascorbate oxidase and ceruloplasmin.