In this handbook, the authors present topical research in the study of the formation mechanisms, physiological roles and common harmful effects of reactive oxygen species (ROS). Topics discussed in this compilation include the role of mitochondria in the generation of ROS; reactive oxygen species in cyanobacteria; participation of ROS in the formation of induced resistances of plants to abiotic stressors; ROS and autoimmune diseases; metal oxide nanoparticles as a source for ROS and their application in the fabrication of antimicrobial textiles; the synergistic effect of photocatalytic water purification with ROS; oxidative stress on neurodegeneration and its implications in Alzheimer’s and Parkinson’s disease; the interplay between ROS and gaseous messenger molecules in plant response to environmental stresses; ROS in non-invasive cancer therapy; oxidative stress in the lungs and blood induced by smoking and exercise; plant signaling networks involving reactive oxygen species and Ca2+; free radicals and the role of antioxidant enzymes on ionizing radiation resistance in zygophyllum fabago, phragmites australis, argusia sibirica L., and eleagnus caspica plants; reactive oxygen species and wound healing; and gender dimorphism in hepatic oxidative stress induced by protein malnutrition during pregnancy and childhood.