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Reactive oxygen species are classically described as occurring as an accidental byproduct of respiration, and are generally thought to be deleterious to biologic systems. The phagocyte nicotinamide adenine dinucleotide phosphate oxidase provides an example of deliberate reactive oxygen species generation, but the function of this enzyme is to oxidatively modify bacteria as part of bactericidal mechanisms. The discovery of a family of nicotinamide adenine dinucleotide (phosphate) oxidases related to the phagocyte oxidase, the Nox/Duox family, provides additional examples of deliberate generation of reactive oxygen species. This article describes this new family of enzymes and considers hypotheses for their function. Potential roles of Nox/Duox in generation of reactive oxygen species that function in cell signaling (related to growth and angiogenesis), immune function, hypoxic response, and oxidative modification of extracellular matrix proteins are discussed.