The generation of reactive oxygen radicals in mammalian cells profoundly affects numerous critical cellular functions, and the absence of efficient cellular detoxification mechanisms which remove these radicals can result in several human diseases. Growing evidence suggests that reactive oxygen species (ROS) within cells act as second messengers in intracellular signaling cascades which induce and maintain the oncogenic phenotype of cancer cells. ROS are tumorigenic by virtue of their ability to increase cell proliferation, survival, cellular migration, and also by inducing DNA damage leading to genetic lesions that initiate tumorigenicity and sustain subsequent tumor progression. However, it is also known that ROS can induce cellular senescence and cell death and can therefore function as anti-tumorigenic agents. Therefore, the mechanisms by which cells respond to reactive oxygen species depends on the molecular background of cell and tissues, the location of ROS production and the concentration of individual ROS species. Carcinoma cells produce ROS at elevated rates in vitro, and in vivo many tumors appear persistent to oxidative stress. Thus, the finding that a diet rich in antioxidants or the elimination of ROS by antioxidant compounds prevents the development of certain cancers provided the setting for subsequent investigation of the tumorigenic actions of reactive oxygen species. This review outlines the current knowledge on the various roles of ROS in tumor development and progression.