This chapter comprehensively examines the biological mechanisms of oxidative stress and its health impacts, with an in-depth analysis of the antioxidant effects of tea bioactive compounds. Oxidative stress arises from an imbalance between free radical production and elimination, triggered by endogenous pathways or exogenous factors, leading to lipid peroxidation, DNA damage, mitochondrial dysfunction, and disrupted signaling cascades. These alterations promote inflammatory responses, cellular senescence, and chronic diseases. Critical antioxidant components in tea—including polyphenols (notably catechins), alkaloids, amino acids, and flavonoids—exhibit structure–activity relationships dependent on phenolic hydroxyl group count/spatial configuration, esterification/glycosylation modifications. Mechanistically, tea polyphenols not only directly scavenge free radicals but also activate the Nrf2/ARE pathway to upregulate antioxidant enzymes, while modulating AKT-JNK and MAPK-NF-κB signaling axes to suppress inflammation and apoptosis. Collectively, tea constituents mitigate oxidative damage through multi-target interactions, establishing a molecular basis for preventing oxidative stress-related pathologies.