Oxidative stress is a biological condition involving an imbalance between reactive oxygen species production and the body’s antioxidant defense systems. Reactive oxygen species are generated during normal cellular metabolism, mitochondrial respiration, immune activity, environmental toxin exposure, ultraviolet radiation exposure, air pollution exposure, smoking, excessive processed food intake, hyperglycemia, chronic inflammation, and metabolic dysfunction. When antioxidant systems become overwhelmed, oxidative damage can affect lipids, proteins, DNA, mitochondria, cell membranes, and intracellular signaling pathways. Mitochondria are major sites of reactive oxygen species production because oxidative phosphorylation continuously transfers electrons through the electron transport chain. Leakage of electrons during mitochondrial respiration can form superoxide radicals and related oxidants. Under normal conditions, endogenous antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase, and peroxiredoxin neutralize excessive oxidants. Nutritional deficiencies, environmental exposures, metabolic dysfunction, inflammatory signaling, and poor dietary patterns may impair these systems and increase cellular oxidative burden. Oxidative stress is associated with chronic inflammation, endothelial dysfunction, impaired insulin signaling, accelerated cellular aging, mitochondrial dysfunction, impaired tissue repair, altered immune signaling, DNA damage accumulation, lipid peroxidation, and abnormal cellular signaling cascades. Persistent oxidative stress may contribute to impaired vascular function, neuronal stress responses, altered glucose metabolism, and cellular structural instability. Oxidative imbalance also affects signaling systems including NF-κB signaling, Nrf2 antioxidant response pathways, FOXO signaling, mitochondrial energy regulation, and DNA repair systems. A whole-food plant-based dietary pattern provides diverse antioxidant compounds, polyphenols, carotenoids, flavonoids, glucosinolates, sulfur compounds, vitamins, minerals, and fiber that support endogenous antioxidant systems. Colorful fruits, cruciferous vegetables, leafy greens, legumes, herbs, spices, mushrooms, seeds, and whole grains contain phytochemicals associated with improved redox balance and cellular resilience. These compounds participate in antioxidant recycling systems, glutathione metabolism, xenobiotic metabolism, mitochondrial protection, endothelial support, and regulation of inflammatory signaling. Foods rich in vitamin C, vitamin E, carotenoids, flavonoids, selenium, magnesium, manganese, and sulfur-containing compounds help support glutathione-related systems and antioxidant enzyme activity. Cruciferous vegetables provide glucoraphanin and sulforaphane associated with Nrf2 pathway activation and detoxification signaling. Berries, green tea, pomegranate, leafy greens, garlic, onions, turmeric, and herbs contain polyphenols linked with antioxidant signaling modulation and reduced oxidative biomarker activity in published research studies.