Hydrogen peroxide emerged as major redox metabolite operative in redox sensing, signaling and redox regulation. Generation, transport and capture of H 2O 2 in biological settings as well as their biological consequences can now be addressed. The present overview focuses on recent progress on metabolic sources and sinks of H 2O 2 and on the role of H 2O 2 in redox signaling under physiological conditions (1–10 nM), denoted as oxidative eustress. Higher concentrations lead to adaptive stress responses via master switches such as Nrf2/Keap1 or NF-κB. Supraphysiological concentrations of H 2O 2 (>100 nM) lead to damage of biomolecules, denoted as oxidative distress. Three questions are addressed: How can H 2O 2 be assayed in the biological setting? What are the metabolic sources and sinks of H 2O 2? What is the role of H 2O 2 in redox signaling and oxidative stress?