Cells use a considerable amount of energy to maintain an internal reducing environment. They synthesize proteins that can scavenge oxidants, chelate redox-active metals and repair oxidatively damaged lipids, proteins, and nucleic acids. Low molecular mass compounds are imported or synthesized to supply electrons to oxidized biomolecules. The complex antioxidant networks are highly coordinated, enabling removal of various oxidants and redox-active compounds, and any intermediates generated during the detoxification of an oxidizing species. Increased oxidative stress initiates a rapid response in cells, with alterations in antioxidant protein expression and diversion of more metabolic energy into reductive processes. The accumulation of oxidatively damaged biomolecules can be a signal to bolster antioxidant defenses, but more importantly, cells appear able to respond to subtle disturbances in homeostasis before damage occurs. This ability to sense and respond to increased oxidant levels is one form of redox signaling.