A microbe’s physiology is geared to work only within a narrow range of physical parameters. But in nature, the environment can quickly change. Many marine microbes, for instance, can move from deep-sea cold to the searing heat of a thermal vent. How do these organisms survive? Stopgap measures called stress survival responses help, but a more permanent solution is when a species slowly evolves to thrive, not just survive, in extreme environments. How does the biology of these so-called extremophiles permit growth under conditions that seem uninhabitable? This chapter explores the limits of microbial growth and shows how this knowledge helps us control the microbial world.