Reactive oxygen species (ROS) are produced by normal cellular metabolism and environmental factors such as air pollutants or cigarette smoke. ROS are highly reactive molecules that can damage cell structures like carbohydrates, nucleic acids, lipids, and proteins, altering their functions. The shift in the balance between oxidants and antioxidants in favor of oxidants is termed "oxidative stress," which can lead to various pathological conditions including cancer, neurological disorders, atherosclerosis, hypertension, diabetes, and respiratory diseases. Aerobic organisms have integrated antioxidant systems, including enzymatic and nonenzymatic antioxidants, to counteract the harmful effects of ROS. However, in pathological conditions, these antioxidant systems can be overwhelmed. This review summarizes the cellular oxidant and antioxidant systems and discusses the cellular effects and mechanisms of oxidative stress. Key topics include the sources of ROS, the role of antioxidants, and the impact of oxidative stress on DNA, lipids, proteins, and signal transduction pathways.Reactive oxygen species (ROS) are produced by normal cellular metabolism and environmental factors such as air pollutants or cigarette smoke. ROS are highly reactive molecules that can damage cell structures like carbohydrates, nucleic acids, lipids, and proteins, altering their functions. The shift in the balance between oxidants and antioxidants in favor of oxidants is termed "oxidative stress," which can lead to various pathological conditions including cancer, neurological disorders, atherosclerosis, hypertension, diabetes, and respiratory diseases. Aerobic organisms have integrated antioxidant systems, including enzymatic and nonenzymatic antioxidants, to counteract the harmful effects of ROS. However, in pathological conditions, these antioxidant systems can be overwhelmed. This review summarizes the cellular oxidant and antioxidant systems and discusses the cellular effects and mechanisms of oxidative stress. Key topics include the sources of ROS, the role of antioxidants, and the impact of oxidative stress on DNA, lipids, proteins, and signal transduction pathways.