Reactive oxygen species and tumor biology are intertwined in a complex web, making it difficult to understand which came first, whether oxidants are required for tumor cell growth, and whether oxidant stress can be exploited therapeutically. Evidence suggests that transformed cells use ROS signals to drive proliferation and other events required for tumor progression. This confers a state of increased basal oxidative stress, making them vulnerable to chemotherapeutic agents that further augment ROS generation or that weaken antioxidant defenses of the cell. In this respect, it appears that tumor cells may die by the same systems they require.