Molecular oxygen is essential for aerobic organisms in order to synthesize large amounts of energy during the process of oxidative phosphorylation and it is harnessed in the form of adenosine triphosphate, the chemical energy of the cell. Oxygen is toxic for anaerobic organisms but it is also less obvious that oxygen is poisonous to aerobic organisms at higher concentrations of oxygen. For instance, oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at increased partial pressures. Reactive oxygen species (ROS) are chemically reactive molecules containing oxygen that are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, in pathological conditions ROS levels can increase dramatically. This may result in significant damage to cell structures. Living organisms have been adapted to the ROS in two ways: they can mitigate the unwanted effects through removal by the antioxidant systems and can advantageously use them as messengers in cell signaling and regulation of body functions. Some other physiological functions of ROS include the regulation of vascular tone, detection, and adaptation to hypoxia. In this review, we describe the mechanisms of oxidative damage and its relationship with the most highly studied neurodegenerative diseases. © 2017 Elsevier Inc. All rights reserved.