The NADPH oxidase (Nox) family of enzymes generates reactive oxygen species (ROS). At low ROS concentration, intracellular signaling is initiated, whereas at high ROS concentration, oxidative stress is induced. The extensive studies over the years have shed light on the mediating roles of the Nox enzymes in a variety of normal physiological processes ranging from bactericidal activity to remodeling of the extracellular matrix. Consequently, imbalance of Nox activities could be the potential cause of acute or chronic diseases. With regard to functional relationships between Nox isoforms and pathogenesis, it is of particular interest to study whether they are involved in carcinogenesis, because overproduction of ROS has long been implicated as a risk factor in cancer development. We see one remarkable example of the causal relationship between Nox1 and cancer in Ras oncogene-induced cell transformation. Other studies also indicate that the Nox family of genes appears to be required for survival and growth of a subset of human cancer cells. Thus, the Nox family will be a focus of attention in cancer biology and etiology over the next couple years.