The history of studies in biology regarding reactive oxygen species (ROS) is approximately 40 years. During the initial 30 years, it appeared that these studies were mainly focused on the toxicity or microbicidal-related agents of ROS. However, recent studies have identified another action regarding oxidative signaling, other than toxicity of ROS. Basically, it is suggested that ROS are reactive, and degenerate to biomacromolecules such as DNA and proteins, leading to deterioration of cellular functions as an oxidative stress. On the other hand, recent studies have shown that ROS act as oxidative signaling in cells, resulting in various gene expressions. For example, NADPH oxidase, a major source of superoxide radicals (O(2)(-)), expresses in various tissues such as leukocytes and cardiovascular systems, and ROS derived from the enzyme play important roles in cell proliferation, differentiation, and cell death. In this review, we have focused on and described the basic properties, toxicity, and roles of ROS.