Significance: Redox pioneer Helmut Sies attempted to explain reactive species' challenges faced by organelles, cells, tissues, and organs via three complementary definitions: (1) oxidative stress, i.e. the disturbance in the prooxidant-antioxidant defense balance in favor of the prooxidants, (2) oxidative eustress, the low physiological exposure to prooxidants, and (3) oxidative distress, the supraphysiological exposure to prooxidants. Recent Advances: Identification, concentration and interactions are the most important elements to improve our understanding of reactive species in physiology and pathology. In this context, the reactive species interactome (RSI) is a new multilevel redox regulatory system that identifies reactive species families, reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species (RSS), and integrates their interactions with their downstream biological targets.Critical issues: We propose a united view to fully combine reactive species identification, oxidative eustress and distress, and the RSI system. In this view, we also propose including the forgotten reactive carbonyl species (RCS), an increasingly rediscovered reactive species family related to the other reactive families, and key enzymes within the RSI. We focus on brain physiology and pathology to demonstrate why this united view should be considered.Future directions: More studies are needed for an improved understanding of the contributions of reactive species through their identification, concentration, and interactions, including in the brain. Appreciating the reactive species interactome in its entirety should unveil new molecular players and mechanisms in physiology and pathology in the brain and elsewhere.