Increasing interest in mononuclear nonheme iron enzymes that activate dioxygen has resulted in an explosion of information on such enzymes in recent years. Concomitantly, efforts to model the active sites of these enzymes have produced synthetic complexes capable of mimicking some aspect of the reactivity of the metal center in several enzymes. These functional models carry out oxidative transformations analogous to those catalyzed by the enzymes and in some cases allow iron(III)-peroxo or iron(IV)-oxo intermediates to be trapped and characterized.