The mitochondrial electron transport chain (ETC) is a critical component of cellular respiration, consisting of complexes I-IV and the electron carriers ubiquinone and cytochrome c. The ETC couples electron transfer with proton gradient generation, which is used by complex V (ATP synthase) to produce ATP. However, some electrons leak from the ETC, leading to the production of reactive oxygen species (ROS), which play dual roles in cellular signaling and damage. ROS are generated at specific sites in each complex, such as sites I_F, I_Q in complex I, II_F in complex II, and III_Q0 in complex III. Excessive ROS can cause cellular damage and disease, while moderate levels are involved in physiological processes like cell proliferation and hypoxia adaptation. Proton leak, regulated by uncoupling proteins (UCPs), is another important aspect of mitochondrial function. Proton leak includes basal and inducible leak, with UCPs playing a key role in modulating proton transport. UCP1 is primarily involved in non-shivering thermogenesis in brown adipose tissue, while UCP2-5 are involved in reducing oxidative stress. Abnormal UCP expression or function is associated with various diseases, including diabetes, obesity, and neurodegenerative disorders. The review discusses the structure and function of each ETC complex, the sites of ROS generation, and the role of UCPs in proton leak and oxidative stress regulation. It highlights the importance of understanding these processes in the context of mitochondrial dysfunction and disease. The study also emphasizes the potential therapeutic applications of targeting UCPs to mitigate oxidative stress and related diseases.The mitochondrial electron transport chain (ETC) is a critical component of cellular respiration, consisting of complexes I-IV and the electron carriers ubiquinone and cytochrome c. The ETC couples electron transfer with proton gradient generation, which is used by complex V (ATP synthase) to produce ATP. However, some electrons leak from the ETC, leading to the production of reactive oxygen species (ROS), which play dual roles in cellular signaling and damage. ROS are generated at specific sites in each complex, such as sites I_F, I_Q in complex I, II_F in complex II, and III_Q0 in complex III. Excessive ROS can cause cellular damage and disease, while moderate levels are involved in physiological processes like cell proliferation and hypoxia adaptation. Proton leak, regulated by uncoupling proteins (UCPs), is another important aspect of mitochondrial function. Proton leak includes basal and inducible leak, with UCPs playing a key role in modulating proton transport. UCP1 is primarily involved in non-shivering thermogenesis in brown adipose tissue, while UCP2-5 are involved in reducing oxidative stress. Abnormal UCP expression or function is associated with various diseases, including diabetes, obesity, and neurodegenerative disorders. The review discusses the structure and function of each ETC complex, the sites of ROS generation, and the role of UCPs in proton leak and oxidative stress regulation. It highlights the importance of understanding these processes in the context of mitochondrial dysfunction and disease. The study also emphasizes the potential therapeutic applications of targeting UCPs to mitigate oxidative stress and related diseases.