Historically, reactive oxygen species (ROS) are thought to exclusively cause cellular damage and physiological
dysfunctions. The accumulation of ROS and oxidative damage has been linked to multiple pathologies, including
neurodegenerative diseases, diabetes, cancer, and premature aging. Accumulating evidence in recent years suggests that
ROS are critical for health. In physiological conditions, ROS are considered as a signaling molecule produced at low level
and in a transient manner. On the other hand, over-production of ROS, associated with the dysfunction of ROS buffering
systems, can lead to the development of chronic diseases. Thus, chronic ROS release participates in the occurrence of over
100 kinds of diseases, including major diseases that threaten human health, such as cancer, cardiovascular diseases, diabetes,
neurodegenerative diseases (Parkinson’s and Alzheimer’s diseases), acquired immune deficiency syndrome, stroke and aging.
This paper reviewes the latest progress in the presence of oxygen on earth and the evolution of aerobic eukaryotic organisms,
ROS types, physiological and pathological pleiotropy of ROS, the major ROS in the body, the generation and reactions of
ROS, the association of ROS with mitochondrial oxidative phosphorylation, apoptosis, immune defense, inflammation and
cancers. Given the importance of ATP and ROS to life, we propose that the balance between the generation of ATP and ROS
by regulation of mitochondrial oxidative phosphorylation will be the key to human health.