NADPH-oxidases (NOXs) are the only known enzymes whose specific function is reactive-oxygen-species (ROS) generation. The catalytic reaction starts with the reduction of the flavin by NADPH. This is followed by a step-wise electron transfer that leads to the reduction of the outer heme, located on the external side of the membrane, where ROS generation takes place. NOX1, NOX2, and NOX3 are regulated by various cellular proteins whereas NOX5 and DUOXs are activated by calcium through additional calcium-binding domains. NOX4 is the only constitutively active member of the family. Generally, NOXs coordinate cell damage, stress responses and tissue regeneration. In particular, NOX2 is at the heart of the innate immunity by eliminating bacterial threats in the phagocytes. High levels of ROS derived from NOXs hyperactivity can result in genetic instability followed by excessive proliferation, activation of the DNA-damage response, proliferative senescence, and apoptosis. With their prototypical roles in ROS biology and redox signalling, NOXs are attractive drug targets in inflammation, fibrotic and degenerative processes, and cancer.