This article reviews the chemical and molecular mechanisms of antioxidants, focusing on experimental approaches and model systems. Free radicals, derived from oxygen, nitrogen, and sulfur molecules, are highly reactive and can cause oxidative stress by damaging DNA, RNA, proteins, and lipids. Antioxidants, both endogenous and dietary, help maintain cellular antioxidant status by directly reacting with free radicals or by inhibiting the activity of free radical-generating enzymes and enhancing the activity of intracellular antioxidant enzymes. The review discusses various methods for measuring antioxidant activity, including radical scavenging assays, stable radical scavenging, and metal ion chelating. It also explores the inhibition of free radical-generating enzymes such as NADPH oxidase and xanthine oxidase, and the activation of internal antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX). The article highlights the importance of natural antioxidants from plant sources and their potential in preventing and treating oxidative stress-related diseases. Additionally, it covers the use of cell-free biological systems and animal models to study the mechanisms and kinetics of antioxidants, emphasizing the role of chemical and cell-free systems in understanding the molecular action of antioxidants.This article reviews the chemical and molecular mechanisms of antioxidants, focusing on experimental approaches and model systems. Free radicals, derived from oxygen, nitrogen, and sulfur molecules, are highly reactive and can cause oxidative stress by damaging DNA, RNA, proteins, and lipids. Antioxidants, both endogenous and dietary, help maintain cellular antioxidant status by directly reacting with free radicals or by inhibiting the activity of free radical-generating enzymes and enhancing the activity of intracellular antioxidant enzymes. The review discusses various methods for measuring antioxidant activity, including radical scavenging assays, stable radical scavenging, and metal ion chelating. It also explores the inhibition of free radical-generating enzymes such as NADPH oxidase and xanthine oxidase, and the activation of internal antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX). The article highlights the importance of natural antioxidants from plant sources and their potential in preventing and treating oxidative stress-related diseases. Additionally, it covers the use of cell-free biological systems and animal models to study the mechanisms and kinetics of antioxidants, emphasizing the role of chemical and cell-free systems in understanding the molecular action of antioxidants.