The perception of the roles of H2O2 in living systems has come a long way, transcending from H2O2 being considered as (i) exclusively damaging; (ii) a necessary evil “unwanted but an inevitable product of aerobic metabolism”; (iii) important for specific biological processes that involve ROS aggressiveness, such as the battle of the innate immune system with pathogens; and (iv) signalling species [1–6]. The list does not end here. A number of studies have illustrated that at concentrations in the high physiological range, H2O2 induces more permanent, modifying changes, adaptations, increasing the resistance of biological systems to the same stimulus (hormesis) or other stressors (cross-adaptation), or enabling the adaptation to altered ecology. The capability of H2O2 to induce the synthesis of a large number of proteins and to provide cross-resistance implies that living systems may intentionally produce H2O2 as a component of adaptation in response to different fluctuations and perturbations shifting the system away from homeostasis [7–9]. Some data even implicate an important role of H2O2 in interspecies communication and in the development of multicellularity [10].