So long as cellular energy generation by enzymatic systems, β-oxidation of fatty acids in the liver peroxisomes and phagocytosis stimulation by pathogens persist, there will be a continuous production of reactive oxygen species in the cell. Reactive oxygen species play important roles in regulating signal transduction processes in the cell. Their physiological or pathological relevance to the cell depends on their concentration, site of production, duration of exposure to cells, and the redox state of the cellular environment. This review examines the physiological role played by reactive oxygen species in cell signaling events, their involvement in the etiology of many diseases like cardiovascular disorders, cancer, oxidative stress and muscle wasting, as well as the mechanisms by which these reactive oxygen species exert their effects. The definition of the role of reactive oxygen species in the aforementioned pathologies may help open the way for opportunities in the development of new drugs targeted toward lowering their concentration and rate of production, or their immediate clearance from the system soon after they are produced.
