Over the last decade, the NADPH oxidases have been recognized as an important source of reactive oxygen species (ROS) involved in both normal physiologic redox-dependent processes, and in the generation of oxidative stress, with a role in the development of cardiovascular disease. As a multi-subunit enzyme complex, the NADPH oxidases have the unusual composition of multiple interchangeable homologs of the catalytic, or Nox (NADPH oxidase), subunit that directly interacts with membrane-bound p22phox. It is at the Nox subunit that NADPH binding leads to electron transfer via heme groups to oxygen resulting in the generation of superoxide anion. Recent investigations have identified seven distinct Nox subunits, some possessing multiple splice variants, but the real challenge has been to characterize the specific cellular functions and the regulation of the various Nox enzymes. The difficulty in doing so is partly due to the presence of multiple Nox isoforms within cells, the low and transient levels of ROS produced with Nox activation, limitations in the ability to detect protein levels of specific Nox enzymes, and the potential for one Nox protein to compensate for the change in expression or activity of a different Nox protein. Although significant progress has been made, it can be argued that our knowledge of the regulation of Nox activity has only partially extended beyond what could be inferred from the model of the phagocyte oxidase (i.e. gp91phox or Nox2), which has been characterized over the past twenty to thirty years.