 Type II diabetes is the most prevalent and serious metabolic disease all over the world, and its hallmarks are pancreatic cell dysfunction and insulin resistance. Under diabetic conditions, chronic hyperglycemia and subsequent augmentation of reactive oxygen species, ROS, deteriorate cell function and increase insulin resistance, which leads to the aggravation of type II diabetes. In addition, chronic hyperglycemia and ROS are also involved in the development of atherosclerosis, which is often observed under diabetic conditions. Taken together, it is likely that ROS play an important role in the development of type II diabetes and atherosclerosis. This article was authored by Hideaki Kaneto, Naoto Cutakami, Munhaid Matsuhisa, and others.