A number of diseases and conditions have been associated with prolonged or persistent exposure to non-physiological levels of reactive oxygen species (ROS). Similarly, ROS underproduction due to loss-of-function mutations in superoxide or hydrogen peroxide (HO)-generating enzymes is a risk factor or causative for certain diseases. However, ROS are required for basic cell functions; in particular the diffusible second messenger HO that serves as signaling molecule in redox processes. This activity sets HO apart from highly reactive oxygen radicals and influences the approach to drug discovery, clinical utility, and therapeutic intervention. Here we review the chemical and biological fundamentals of ROS with emphasis on HO as a signaling conduit and initiator of redox relays and propose an integrated view of physiological versus non-physiological reactive species. Therapeutic interventions that target persistently altered ROS levels should include both selective inhibition of a specific source of primary ROS and careful consideration of a targeted pro-oxidant approach, an avenue that is still underdeveloped. Both strategies require attention to redox dynamics in complex cellular systems, integration of the overall spatiotemporal cellular environment, and target validation to yield effective and safe therapeutics. The only professional primary ROS producers are NADPH oxidases (NOX1-5, DUOX1-2). Many other enzymes, e.g., xanthine oxidase (XO), monoamine oxidases (MAO), lysyl oxidases (LO), lipoxygenase (LOX), and cyclooxygenase (COX), produce superoxide and HO secondary to their primary metabolic function. Superoxide is too reactive to disseminate, but HO is diffusible, only limited by adjacent PRDXs or GPXs, and can be apically secreted and imported into cells through aquaporin (AQP) channels. HO redox signaling includes oxidation of the active site thiol in protein tyrosine phosphatases, which will inhibit their activity and thereby increase tyrosine phosphorylation on target proteins. Essential functions include the oxidative burst by NOX2 as antimicrobial innate immune response; gastrointestinal NOX1 and DUOX2 generating low HO concentrations sufficient to trigger antivirulence mechanisms; and thyroidal DUOX2 essential for providing HO reduced by TPO to oxidize iodide to an iodinating form which is then attached to tyrosyls in TG. Loss-of-function (LoF) variants in TPO or DUOX2 cause congenital hypothyroidism and LoF variants in the NOX2 complex chronic granulomatous disease.