This book presents information about the high enzymatic reactivity of reactive oxygen species. Chapters in the book cover several aspects of the topic such as the sources, formation mechanisms, reaction centers, oxidation-reduction reactions, cellular respiration chemistry, enzymatic kinetics, mitochondrial and plastid electron transport chains, oxidation-reduction potential, reaction constants, reaction velocity and reaction mechanisms involved, cellular cytotoxicity, antioxidant defense mechanisms in plants and animals, response of plants to conditions of environmental stress, xenobiotics, and the thermodynamics inherent to oxygen metabolism. The book also features a chapter on flavonoids which highlights a paradoxical facet of the affinity of reactive oxygen species for enzymes. Flavonoids are mainly antioxidant molecules as they act as trappers of reactive oxygen species. The chapter informs readers about the metabolic pathways mediated by enzymes through wich flavonoids become promoters of these same reactive oxygen species. All chapters present the subject in a simple, analytical format, while highlighting the scientific evidence gathered by researchers so far. The volume is an interesting reference for scholars learning about the biochemistry and enzymology of oxygen and its free radical derivatives.