Oxidative stress is caused by an imbalance in the redox status of the body. In such a state, increase of freeradicals in the body can lead to tissue damage. One of the most important species of free radicals is reactiveoxygen species (ROS) produced by various metabolic pathways, including aerobic metabolism in the mitochondrialrespiratory chain. It plays a critical role in the initiation and progression of various types of cancers. ROS affectsdifferent signaling pathways, including growth factors and mitogenic pathways, and controls many cellularprocesses, including cell proliferation, and thus stimulates the uncontrolled growth of cells which encourages thedevelopment of tumors and begins the process of carcinogenesis. Increased oxidative stress caused by reactivespecies can reduce the body’s antioxidant defense against angiogenesis and metastasis in cancer cells. Theseprocesses are main factors in the development of cancer. Bimolecular reactions cause free radicals in whichcreate such compounds as malondialdehyde (MDA) and hydroxyguanosine. These substances can be used asindicators of cancer. In this review, free radicals as oxidizing agents, antioxidants as the immune system, andthe role of oxidative stress in cancer, particularly breast cancer, have been investigated in the hope that betteridentification of the factors involved in the occurrence and spread of cancer will improve the identification oftreatment goals.