An imbalance in the redox state of the cells, which may be caused by the overproduction of reactive oxygen species (ROS) or dysfunction of the antioxidant systems leads to the state of oxidative stress. In this state, the capability of the biological system to detoxify the reactive products is reduced. Increased ROS levels have negative effects on essential cellular structures, such as nucleic acids, proteins and lipids. Oxidative stress may cause development of several diseases, including Parkinson’s disease, chronic fatigue syndrome (ME/CFS), cancer, depression, Alzheimer’s disease, heart failure, fragile X syndrome and Lafora disease. Numerous antioxidants including polyphenols, vitamin E, and flavonoids have been studied for their potential properties against oxidative stress. From theories to research to practical applications, studies related to all contemporary topics of relevance on the mechanisms and health impacts of oxidative stress have been included in this book. The readers would gain knowledge that would broaden their perspective in this area of study.