The free radical-coupled copper catalytic motif has emerged as the unifying feature of a new family of enzymes, the radical copper oxidases. Their highly evolved active sites include a novel amino acid modification, the Tyr-Cys dimer, that forms spontaneously through self-processing of the protein during its maturation. The active site is remarkable in the extent to which metal ligands participate in the catalytic process. Rather than simply coordinating the metal ion, the ligands perform essential redox and proton-transfer functions in the chemistry of the active site, directed by their interactions with the copper center in the protein. The wide phylogenetic distribution and range of functions represented within the family hint of a fundamental role for these enzymes in the biology of oxygen. The roles for these enzymes are further expanding through a variety of biotechnological applications.