The mammalian mitochondrial electron transport chain (ETC) consists of complexes I-IV and electron transporters ubiquinone and cytochrome c. The ETC generates a proton gradient across the inner membrane, which is used by complex V (ATP synthase) to produce ATP. Some electrons are directly transferred to oxygen, generating reactive oxygen species (ROS). This review discusses the sites of ROS generation in each ETC complex, including sites Iₕ and I₀ in complex I, site IIₕ in complex II, and site III₀ in complex III, and the physiological and pathological regulation of ROS. ROS play important roles in cell proliferation, hypoxia adaptation, and cell fate determination but can also cause irreversible cell damage and death. Proton leak and uncoupling proteins (UCPs) are also discussed. Proton leak consists of basal and induced proton leak, with UCPs playing a crucial role in the regulation of induced proton leak. Five UCPs (UCP1-5) have been identified in mammalian cells, with UCP1 primarily involved in maintaining body temperature in cold environments through non-shivering thermogenesis, and UCP2-5 reducing oxidative stress under certain conditions, exerting cytoprotective effects. Diseases involving oxidative stress are associated with UCPs. The review concludes by emphasizing the importance of understanding electron transfer in the ETC and the mechanisms of electron and proton leaks for both physiological and pathological processes.The mammalian mitochondrial electron transport chain (ETC) consists of complexes I-IV and electron transporters ubiquinone and cytochrome c. The ETC generates a proton gradient across the inner membrane, which is used by complex V (ATP synthase) to produce ATP. Some electrons are directly transferred to oxygen, generating reactive oxygen species (ROS). This review discusses the sites of ROS generation in each ETC complex, including sites Iₕ and I₀ in complex I, site IIₕ in complex II, and site III₀ in complex III, and the physiological and pathological regulation of ROS. ROS play important roles in cell proliferation, hypoxia adaptation, and cell fate determination but can also cause irreversible cell damage and death. Proton leak and uncoupling proteins (UCPs) are also discussed. Proton leak consists of basal and induced proton leak, with UCPs playing a crucial role in the regulation of induced proton leak. Five UCPs (UCP1-5) have been identified in mammalian cells, with UCP1 primarily involved in maintaining body temperature in cold environments through non-shivering thermogenesis, and UCP2-5 reducing oxidative stress under certain conditions, exerting cytoprotective effects. Diseases involving oxidative stress are associated with UCPs. The review concludes by emphasizing the importance of understanding electron transfer in the ETC and the mechanisms of electron and proton leaks for both physiological and pathological processes.