The reactive oxygen species (ROS) can damage the nucleic acids. The oxidative modification of the DNA constitutes the fundamental molecular event in carcinogenesis and that is why the interest in the study of the involvement of ROS in that process. On the other hand, oxidative DNA damage-induced mutagenesis is widely hypothesized to be a frequent event in the normal human cell. The enormous evidence suggests an important role of ROS in the expansion and progression of tumor clones, being considered a relevant class of carcinogens. In addition, the use of immunohistochemical techniques has showed that the various types of cancer examined to date manifest an imbalance in their antioxidant mechanisms to respect the primary cell. In the near future new insights in cancer therapies, based on modulation of cellular redox status, may lead the way to additional tools against carcinogenesis from ROS.