Book excerpt: The fine balance in reactive species (RS) levels inside the cell, or redox homeostasis, is crucial for normal life. RS are molecules such as hydroxyl, superoxide, and others whose reaction rates are typically high. Further, RS are now accepted as the molecular mediators of stresses on cells. RS are also implicated in many diseases including cancer, cardiovascular diseases, and neurodegenerative diseases.In contrast to the popular view that RS are deleterious, this book discusses desirable outcomes from the manipulation of reactive species levels in the cells. The discoveries include a fundamental approach to overcome major limitations with traditional oxygen supply to bioreactors, a mere treatment of the inoculum to significantly (sometimes many-fold) increase productivity of bio-oil, enzymes, and other products from bioprocesses, unexpected effect of shear on cultivated cells, rhythms in intracellular reactive species levels and resetting the rhythm to significantly improve cancer drug efficiency.The book concludes with a description of a scalable reactive species module — one that can be integrated with any genome scale model to gain significant insights into the RS effects on metabolism.