Reactive Oxygen Species, or ROS, are a potentially damaging biochemical byproduct of multiple cellular pathways that involve redox reactions, with the most prominent contributors being the energy pathways facilitated by the coenzymes NAD+ and NADP. The reduced forms of these coenzymes are used for detoxification of ROS, and the various ROS themselves are used in a temporary fashion for intra- and inter-cellular signaling. There are currently ~400 known redox reactions in the cell that involve NAD+/NADH, and another ~30 for the phosphorylated variation NADP/NADPH. The far-reaching influence of these molecules makes their measurement and visualization useful even in seemingly unrelated experiments. 

For more information about biochemical measurements of cellular health, see 'Additional Resources' below.