The chapter discusses the role of mitochondria in regulating cell death, particularly apoptosis, through the generation and effects of reactive oxygen species (ROS). Mitochondria are the primary source of ROS, which are mainly produced at Complex I and III of the respiratory chain. Excessive ROS production can lead to oxidative damage to macromolecules, including DNA, proteins, and lipids, and has been implicated in various cellular processes such as mtDNA mutations, aging, and cell death. The release of cytochrome c and other pro-apoptotic proteins from mitochondria is a key step in the activation of apoptosis, triggered by the dissociation of cytochrome c from cardiolipin, which anchors it to the inner mitochondrial membrane. Mitochondrial antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (Gpx), and peroxiredoxins (Prx), play crucial roles in protecting cells from oxidative stress and preventing apoptosis. The chapter also highlights the importance of cardiolipin in maintaining the stability of the mitochondrial membrane and its role in the release of cytochrome c during apoptosis. Additionally, it explores the impact of oxidative stress on the regulation of apoptosis, including the involvement of pro-oxidant molecular modulators like p66Shc and p53, and the protective effects of antioxidant enzymes. Overall, the chapter emphasizes the critical role of mitochondria in both the generation and defense against ROS, and their significance in the regulation of cell death.The chapter discusses the role of mitochondria in regulating cell death, particularly apoptosis, through the generation and effects of reactive oxygen species (ROS). Mitochondria are the primary source of ROS, which are mainly produced at Complex I and III of the respiratory chain. Excessive ROS production can lead to oxidative damage to macromolecules, including DNA, proteins, and lipids, and has been implicated in various cellular processes such as mtDNA mutations, aging, and cell death. The release of cytochrome c and other pro-apoptotic proteins from mitochondria is a key step in the activation of apoptosis, triggered by the dissociation of cytochrome c from cardiolipin, which anchors it to the inner mitochondrial membrane. Mitochondrial antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (Gpx), and peroxiredoxins (Prx), play crucial roles in protecting cells from oxidative stress and preventing apoptosis. The chapter also highlights the importance of cardiolipin in maintaining the stability of the mitochondrial membrane and its role in the release of cytochrome c during apoptosis. Additionally, it explores the impact of oxidative stress on the regulation of apoptosis, including the involvement of pro-oxidant molecular modulators like p66Shc and p53, and the protective effects of antioxidant enzymes. Overall, the chapter emphasizes the critical role of mitochondria in both the generation and defense against ROS, and their significance in the regulation of cell death.