Redox reactions of aerobic metabolism are directly or indirectly linked to atmospheric oxygen, a reactant which is essentially present in unlimited amounts nowadays. The accumulation of O2 was the consequence of the successful colonization by photoautotrophic organisms of the terrestrial and aqueous habitats during evolution. Concomitantly with the development of oxygenic photosynthesis, oxygen became available as oxidizing reactant in chemical reactions, for instance as terminal electron acceptor of the respiratory electron transport chain, of xanthin oxidase, of lipoxigenase and in photorespiratory oxygenation of ribulose- 1,5-bisphosphate. Although still subject to some controversial discussion, photorespiratory energy consumption in the chloroplasts and amino acid synthesis in the peroxisomes and mitochondria may constitute beneficial or even essential metabolic pathways of plants under certain growth conditions.