This review discusses the generation, damaging reactions, and mechanisms of action of natural antioxidants in combating reactive oxygen species (ROS) and reactive nitrogen species (RNS). ROS and RNS are harmful byproducts of normal cellular functions and environmental exposure, leading to oxidative stress and various diseases. Antioxidants, both enzymatic and non-enzymatic, help neutralize these radicals and prevent cellular damage. Enzymatic antioxidants include superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSHPx), while non-enzymatic antioxidants include vitamins, bioflavonoids, carotenoids, hydroxycinnamates, and other natural compounds. The review highlights the mechanisms by which these antioxidants scavenge free radicals, such as through one-electron transfer reactions, and their roles in preventing oxidative damage to DNA, proteins, and lipids. It also discusses in vitro methods for evaluating antioxidant activity, including superoxide anion radical scavenging, DPPH free radical scavenging, and phospholipid peroxidation assays. The review emphasizes the importance of understanding antioxidant mechanisms for developing effective antioxidants and improving human health. The scope is limited to natural antioxidants and in vitro assays for evaluating their antioxidant properties.This review discusses the generation, damaging reactions, and mechanisms of action of natural antioxidants in combating reactive oxygen species (ROS) and reactive nitrogen species (RNS). ROS and RNS are harmful byproducts of normal cellular functions and environmental exposure, leading to oxidative stress and various diseases. Antioxidants, both enzymatic and non-enzymatic, help neutralize these radicals and prevent cellular damage. Enzymatic antioxidants include superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSHPx), while non-enzymatic antioxidants include vitamins, bioflavonoids, carotenoids, hydroxycinnamates, and other natural compounds. The review highlights the mechanisms by which these antioxidants scavenge free radicals, such as through one-electron transfer reactions, and their roles in preventing oxidative damage to DNA, proteins, and lipids. It also discusses in vitro methods for evaluating antioxidant activity, including superoxide anion radical scavenging, DPPH free radical scavenging, and phospholipid peroxidation assays. The review emphasizes the importance of understanding antioxidant mechanisms for developing effective antioxidants and improving human health. The scope is limited to natural antioxidants and in vitro assays for evaluating their antioxidant properties.