Reactive Oxygen Species (ROS) are a class of signaling molecules that regulate intracellular signaling cascades in response to external stimuli. Once accumulated in cells, they can damage DNA modifying gene transcription and affecting protein expression and function in ways that accelerate tumorigenesis. In cancer cells, the accumulation of ROS can increase cell proliferation and cell invasion into other tissues, while, antioxidant enzymes and molecules can protect cells from oxidative stress so as to maintain cellular homeostatic redox status. Cancer cells often do not have sufficient levels of antioxidant enzymes which are needed to rescue cells from oxidative stress. The redox status of cancer cells appears to be a key factor in maintaining the malignant phenotype. Cancer stem cells, on the other hand, have been shown to maintain low levels of ROS in order to retain their self renewal and differentiation potential, even though the exact mechanism is not known yet. ROS and antioxidant enzymes are novel targets for developing anti-cancer therapeutics. In this review, the current understanding for redox regulation of cancer cells and neoplastic stem cells as well as the role and function of anti-oxidant enzymes and molecules is discussed.