Reactive oxygen species (ROS) exhibit different spatial and temporal distributions as well as concentrations in-and outside the cell, thereby functioning as signaling or pathogen-destroying molecules. Especially the ROS H2O2 is important for the patho/physiological status of an organism. Electrochemistry (EM) and electron spin resonance (ESR)-based techniques allow quantification of H2O2 in artificial and living systems, coping a concentration range from low nM up to mM. Working electrodes for EM are optimized by diverse modifications and, additionally, redox mediators are used. Ultramicroelectrodes allow scanning of single cells to spatially resolve and quantify extracellular H2O2 in real-time. With ESR spectroscopy, O-center dot(2)- , but not H2O2, can be directly determined by spin probes in-and outside of cells in suspensions. Monitoring H2O2 requires formation of intermediate radicals, detectable with spin probes. Low mu M [H2O2] can thus be assessed specifically. Using suitable spin traps, in-vivo ESR and immuno-spin trapping can visualize different radicals at their respective production sites in small animals, organs and tissues. Here, the redox reaction cascades may interfere with cell metabolism. Optimization of all methods established for H2O2 determination would be favorable to finally combine them for mutual validation. Thus, a deeper insight into cellular ROS metabolism can be obtained. (C) The Author(s) 2019. Published by ECS.