Hydrogen peroxide (H2O2) is a highly reactive compound produced by cells after secondary reactions as a result of the leakage of electrons from the electron transport chain in mitochondria or as a consequence of other enzymatic reactions. This book examines the detection, applications and health implications of hydrogen peroxide. Topics discussed include the ambient conditions of hydrogen peroxide advanced oxidation processes for wastewater treatment; the electrochemical approach to quantify cellular hydrogen peroxide and monitor its release process from living cells; H2O2 signalling and role in inflammation; H2O2 as a useful oxidative stressor to investigate various intracellular events under pathological conditions; uses of hydrogen peroxide in aquaculture; H2O2 in regulating fungal viability and pathogenicity; massive production of H2O2 upon ATP synthase activation; and H2O2 based green oxidation reactions.