Oxygen is converted into reactive oxygen (RO) by radiation, light, the electron transport system in mitochondria, or by other enzymes and is regulated by the action of antioxidative enzymes which convert RO into an inactive state. Reactive oxygen species (ROS) have a biocidal effect on invading bacteria and they can also injure the cells of the host. For this reason, RO is considered as a general cause of aging and contributes to lifestyle-related diseases and cancer. However, for any organism that uses oxygen as an energy source, RO is inevitably produced and has important biological significance. Apart from the direct activity of RO, recent studies have shown that it functions as a second messenger of signal transduction. In this review, the recent findings related to ROS/nitric oxide (NO) and especially of its relationship to innate immunity are summarized.