Description: Longevity and aging are two sides of a coin, leaving the question open as to which one is the cause and which one the effect. At the individual level, the physiological rate of aging determines the length of life (= individual longevity, as long as death results from old age and not from disease or other impacts). Individual longevity depends on the direct influence of environmental conditions with respect to nutrition, and the possibility for and timing of reproduction, as well as on the energetic costs animals invest in behavioural and physiological stress defense. All these environmental effectors influence hormonal and cellular signalling pathways that modify the individual physiological condition, the reproductive strategy, and the rate of aging. At the species level, longevity (= maximum lifespan, MLSP) is the result of an evolutionary process and, thus, largely determined by the species’ behavioural and physiological adaptations to its ecological niche. Specifically, reproductive and breeding strategies have to be optimized in relation to local environmental conditions in different habitats. As a result of adaptive and evolutionary processes, species longevity is genetically underpinned, not necessarily by a few aging genes, but by an evolutionary process that has hierarchically shaped and optimized speciesʼ genomes to function in a specific niche or environmental system. Importantly, investigations and reviews attempting to unravel the mechanistic basis of the aging process need to differentiate clearly between the evolutionary process shaping longevity at the species level and the regulatory mechanisms that alter the individual rate of aging.