(Top) Summary of the primary cell site of ROS generation and the major clearance mechanisms for ROS. (Bottom) Our speculation that dependent on the levels of ROS generated and its compartmentalization in the cell ROS could either induce critical carcinogenic events (arrow with + sign) or alternatively play a more dominant role in tumor cell killing (arrow with the – sign). More work is needed to clearly define the level and type of ROS in tumor cells, their innate clearance mechanism(s), and the levels and species of ROS that can be manipulated to kill tumor cells. Illustration by Kenneth Probst.