Molecular oxygen is indispensable for aerobic organisms, as the final acceptor of electrons derived from the normal cellular metabolism at the end of the mitochondrial respiratory chain. However, oxygen also is toxic under some con -ditions. The “oxygen paradox” is that oxygen is dangerous to the same life-forms for which it has become an essential component of energy production. Oxygen toxicity may be explained by the formation of reactive oxygen species (ROS) (including oxygen free radicals) which may oxidize biological macromolecules when there is an excessive ROS production and/or when the antioxidant systems are decreased. Excessive ROS production is associated with several pathological diseases. This review provides a basic overview of the ROS chemistry and the antioxidant systems and will introduce the reader to the most important concepts of this broad topic of study.