Although the amount of research on copper amine oxidases has grown rapidly and substantially in the past decade, the field unfortunately suffers from lack of cohesion and significant confusion surrounds aspects as simple as confirmation of enzyme identities. This book describes the structure of the enzymes, the role of copper, and of the unusual cofactor 6-hydroxydopa quinine derived from a posttranslational modification of a tyrosine residue. It also covers the differences of between AOs from bacteria, plants, and mammals. Finally, the text examines the importance of this ubiquitous class of enzymes in physiology and in metabolism of biogenic amines.