Mitochondria play a crucial role in cellular functions such as bioenergetics, metabolism, biosynthesis, and cell death or survival. Reactive oxygen species (ROS) generated by mitochondria are involved in stress signaling in normal cells but can also contribute to the initiation of nuclear or mitochondrial DNA mutations that promote neoplastic transformation. In cancer cells, mitochondrial ROS amplify the tumorigenic phenotype and accelerate the accumulation of additional mutations leading to metastatic behavior. While disabling mitochondrial function is not a feasible therapy due to their essential roles in normal cells, targeting mitochondria-to-cell redox communication represents a promising avenue for future therapy. The relationship between mitochondria and host cells evolved over billions of years, with mitochondria contributing to cellular energy production and receiving nutrients and protection from extreme pH conditions. Modern mitochondria participate in various functions, including biosynthesis, regulation of calcium ion concentrations, and redox status. Under normal conditions, mitochondria trigger redox signaling through the release of ROS from the electron transport chain (ETC). In cancer, excessive ROS generation can disrupt cellular function by causing lipid, protein, and DNA oxidation. Cancer cells rely on increased mitochondrial ROS signaling to regulate their phenotype, making them vulnerable to therapies that further stress their redox homeostasis. This vulnerability presents both a challenge and an opportunity for novel therapies. The sources of mitochondrial ROS include the tricarboxylic acid cycle (TCA cycle) and the ETC. Enzymes in the TCA cycle and ETC can generate ROS through electron transfers and interactions with O2. Antioxidant systems, such as superoxide dismutases and glutathione peroxidase, help scavenge ROS to limit their signaling effects and prevent oxidative damage. Hypoxia and ROS generation are closely linked in cancer. During hypoxia, non-specific ROS generation decreases, but ROS release from complex III paradoxically increases. This enhanced ROS generation in the hypoxic tumor microenvironment promotes growth, metabolic reprogramming, and survival. Mitochondrial DNA (mtDNA) mutations are important targets of mitochondrial ROS, as they can promote tumorigenesis. Mutations in mtDNA can alter the function of the ETC or ATP synthase, leading to shifts in mitochondrial membrane potential and redox status. These changes can introduce shifts in function that alter ETC redox status and ROS generation, potentially amplifying the tumorigenic phenotype. Excessive mitochondrial oxidant stress can induce cell death, either through apoptosis or the mitochondrial permeability transition pore (MPTP). Low levels of mitochondrial ROS have crucial roles in signaling and protein function regulation, affecting protein thiol oxidation and phosphatase activity. Oncogenic transformation often involves a shift in cytosolic thiol redox balance, enhancing the proliferative phenotype. ROS can also contribute to genomic instability by causing DNA damage and promoting the activation of transcription factors like HIF. In conclusion, mitochondrial ROS are initiators, amplifiers, and potential vulnerabilities inMitochondria play a crucial role in cellular functions such as bioenergetics, metabolism, biosynthesis, and cell death or survival. Reactive oxygen species (ROS) generated by mitochondria are involved in stress signaling in normal cells but can also contribute to the initiation of nuclear or mitochondrial DNA mutations that promote neoplastic transformation. In cancer cells, mitochondrial ROS amplify the tumorigenic phenotype and accelerate the accumulation of additional mutations leading to metastatic behavior. While disabling mitochondrial function is not a feasible therapy due to their essential roles in normal cells, targeting mitochondria-to-cell redox communication represents a promising avenue for future therapy. The relationship between mitochondria and host cells evolved over billions of years, with mitochondria contributing to cellular energy production and receiving nutrients and protection from extreme pH conditions. Modern mitochondria participate in various functions, including biosynthesis, regulation of calcium ion concentrations, and redox status. Under normal conditions, mitochondria trigger redox signaling through the release of ROS from the electron transport chain (ETC). In cancer, excessive ROS generation can disrupt cellular function by causing lipid, protein, and DNA oxidation. Cancer cells rely on increased mitochondrial ROS signaling to regulate their phenotype, making them vulnerable to therapies that further stress their redox homeostasis. This vulnerability presents both a challenge and an opportunity for novel therapies. The sources of mitochondrial ROS include the tricarboxylic acid cycle (TCA cycle) and the ETC. Enzymes in the TCA cycle and ETC can generate ROS through electron transfers and interactions with O2. Antioxidant systems, such as superoxide dismutases and glutathione peroxidase, help scavenge ROS to limit their signaling effects and prevent oxidative damage. Hypoxia and ROS generation are closely linked in cancer. During hypoxia, non-specific ROS generation decreases, but ROS release from complex III paradoxically increases. This enhanced ROS generation in the hypoxic tumor microenvironment promotes growth, metabolic reprogramming, and survival. Mitochondrial DNA (mtDNA) mutations are important targets of mitochondrial ROS, as they can promote tumorigenesis. Mutations in mtDNA can alter the function of the ETC or ATP synthase, leading to shifts in mitochondrial membrane potential and redox status. These changes can introduce shifts in function that alter ETC redox status and ROS generation, potentially amplifying the tumorigenic phenotype. Excessive mitochondrial oxidant stress can induce cell death, either through apoptosis or the mitochondrial permeability transition pore (MPTP). Low levels of mitochondrial ROS have crucial roles in signaling and protein function regulation, affecting protein thiol oxidation and phosphatase activity. Oncogenic transformation often involves a shift in cytosolic thiol redox balance, enhancing the proliferative phenotype. ROS can also contribute to genomic instability by causing DNA damage and promoting the activation of transcription factors like HIF. In conclusion, mitochondrial ROS are initiators, amplifiers, and potential vulnerabilities in