The complex system of molecular communications underlying cell biochemistry and function involves numerous components including kinases, phosphatases and transcription factors, which are known to be sensitive to cellular and tissue redox changes. Reactive oxygen species (ROS), whose constitutive generation in cells and tissues is amplified under pro-oxidant conditions, are now unanimously recognized to be important triggers and modulators of cell signaling, and consequently of cell behavior. This review considers the major signaling pathways that mediate gene regulation in response to ROS.