Contents Summary 1197 I. Introduction 1198 II. Measurement and imaging of H<sub>2</sub> O<sub>2</sub> 1198 III. H<sub>2</sub> O<sub>2</sub> and O<sub>2</sub><sup>·-</sup> toxicity 1199 IV. Production of H<sub>2</sub> O<sub>2</sub> : enzymes and subcellular locations 1200 V. H<sub>2</sub> O<sub>2</sub> transport 1205 VI. Control of H<sub>2</sub> O<sub>2</sub> concentration: how and where? 1205 VII. Metabolic functions of H<sub>2</sub> O<sub>2</sub> 1207 VIII. H<sub>2</sub> O<sub>2</sub> signalling 1207 IX. Where next? 1209 Acknowledgements 1209 References 1209 SUMMARY: Hydrogen peroxide (H<sub>2</sub> O<sub>2</sub> ) is produced, via superoxide and superoxide dismutase, by electron transport in chloroplasts and mitochondria, plasma membrane NADPH oxidases, peroxisomal oxidases, type III peroxidases and other apoplastic oxidases. Intracellular transport is facilitated by aquaporins and H<sub>2</sub> O<sub>2</sub> is removed by catalase, peroxiredoxin, glutathione peroxidase-like enzymes and ascorbate peroxidase, all of which have cell compartment-specific isoforms. Apoplastic H<sub>2</sub> O<sub>2</sub> influences cell expansion, development and defence by its involvement in type III peroxidase-mediated polymer cross-linking, lignification and, possibly, cell expansion via H<sub>2</sub> O<sub>2</sub> -derived hydroxyl radicals. Excess H<sub>2</sub> O<sub>2</sub> triggers chloroplast and peroxisome autophagy and programmed cell death. The role of H<sub>2</sub> O<sub>2</sub> in signalling, for example during acclimation to stress and pathogen defence, has received much attention, but the signal transduction mechanisms are poorly defined. H<sub>2</sub> O<sub>2</sub> oxidizes specific cysteine residues of target proteins to the sulfenic acid form and, similar to other organisms, this modification could initiate thiol-based redox relays and modify target enzymes, receptor kinases and transcription factors. Quantification of the sources and sinks of H<sub>2</sub> O<sub>2</sub> is being improved by the spatial and tempora...