Oxidative stress is the result of an imbalance between the production of reactive oxygen species (ROS) and antioxidant mechanisms. It is characterized by damage of all cellular components, DNA, proteins, lipids. ROS are nevertheless important for the physiology of an organism, as they are involved in the innate immune defense and several intracellular signaling pathways. They play an important role in tumorigenesis by promoting tumor vasculature, which is essential to their growth and metastatic processes. There are many sources of ROS in the cells, but the NOX enzymes (NADPH oxidase-dependent) are now recognized to have a major role in the oxidative stress process. Indeed, they are present in many tissues where their only function is to produce ROS. This article discusses the NOX in endothelial cells and their role in the tumor angiogenesis.