Whereas the majority of O-2-metabolizing enzymes depend on transition metal ions or organic cofactors for catalysis, a significant number of oxygenases and oxidases neither contain nor require any cofactor. Among the cofactor-independent oxidases, urate oxidase, coproporphyrinogen oxidase, and formylglycine-generating enzyme are of mechanistic as well as medical interest. Formylglycine-generating enzyme is also a promising tool for protein engineering as it can be used to equip proteins with a reactive aldehyde function. PqqC, an oxidase in the biosynthesis of the bacterial cofactor pyrroloquinoline quinone, catalyzes an eight-electron ring-closure oxidation reaction. Among bacterial oxygenases, quinone-forming monooxygenases involved in the tailoring of polyketides, the dioxygenase DpgC found in the biosynthesis of a building block of vancomycin and teicoplanin antibiotics, luciferase monooxygenase from Renilla sp., and bacterial ring-cleaving 2,4-dioxygenases active towards 3-hydroxy-4(1H)-quinolones have been identified as cofactor-independent enzymes. Interestingly, the 3-hydroxy-4(1H)-quinolone 2,4-dioxygenases as well as Renilla luciferase use an alpha/beta-hydrolase architecture for oxygenation reactions. Cofactor-independent oxygenases and oxidases catalyze very different reactions and belong to several different protein families, reflecting their diverse origin. Nevertheless, they all may share the common mechanistic concept of initial base-catalyzed activation of their organic substrate and "substrate-assisted catalysis.".