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Cancer development is known by summative action of multiple events occurring in single cell. It can be characterized by three stages: Initiation, promotion, and progression. ROS is involved in all these phases. The effect of oxidative stress at a certain phase of carcinogenesis is directly proportionate to the type and the reactivity of radicals involved. Initiation results when a normal cell sustains a DNA mutation that, when preceded by a round of DNA production, leads to fixation of the mutation, producing an initiated cell. Initiation of malignancy by ROS is supported by presence of oxidative DNA modifications in cancer tissues. The promotion stage is known by clonal expansion of initiated cells, by stimulation of cell proliferation and/or inhibition of apoptosis. Oxidative stress is strongly involved in this phase. ROS can induce expansion of mutated cell clones by temporarily modulating the genes which are associated …