Reactive oxygen species (ROS) are involved in both human health and disease. ROS are produced during normal metabolism. Overproduction of ROS is called “oxidative stress”. Free radicals and other oxidants cause oxidation of lipids, proteins and DNA, thereby increasing the likelihood of tissue injury. Toxic products of radicals’ reactions exert cytostatic effects, cause membrane damage and activate pathways of cell death. The redox status of the cell is maintained by antioxidant enzymes and substances such as glutathione, vitamins E, C and A, and thioredoxin, which serve to remove ROS. Lowering oxidative stress can have a clinical benefit, whereas the excess production of ROS is either the primary cause of diseases or secondary complication. Oxidative stress diseases include cardiovascular, neurodegenerative and inflammatory diseases, such as rheumatoid arthritis. Therefore, knowledge about oxidative stress and its biomarkers may be very useful.