Elevation of the level of intracellular reactive oxygen species (ROS) has immense implication in the biological system. On the one hand, ROS promote the signaling cascades for the maintenance of normal physiological functions, the phenomenon referred to as redox biology, and on the other hand increased ROS can cause damages to the cellular macromolecules as well as genetic material, the process known as oxidative stress. Oxidative stress acts as an etiological factor for wide varieties of pathologies, cancer being one of them. ROS is regarded as a “double-edged sword” with respect to oncogenesis. It can suppress as well as promote the malignant progression depending on the type of signaling pathway it uses. Moreover, the attribution of ROS in promoting phenotypic plasticity as well as acquisition of stemness during neoplasia has become a wide area of research. The current review discussed all the aspects of ROS in the perspective of tumor biology with special reference to epithelial-mesenchymal transition (EMT) and cancer stem cells.

细胞内活性氧（ROS）水平的升高在生物系统中具有巨大的意义。一方面，ROS促进维持正常生理功能的信号级联反应，这种现象被称为氧化还原生物学，另一方面，ROS的增加可导致细胞大分子以及遗传物质的破坏，这一过程称为氧化应激。氧化应激是多种病理的病因，癌症就是其中之一。ROS被认为是致癌作用的“双刃剑”。它可以抑制并促进恶性进展，具体取决于其使用的信号传导途径的类型。此外，ROS在促进表型可塑性以及在瘤形成过程中获得茎干的作用已成为广泛的研究领域。本综述从肿瘤生物学的角度讨论了ROS的所有方面，并特别提及了上皮-间质转化（EMT）和癌症干细胞。