A limiting factor for the survival of life in a changing environment is the intracellular production of reactive oxygen species. These can damage the building blocks of life (DNA, proteins, lipids) through oxidation. All organisms, including microbial extremophiles, have developed mechanisms to quench the reactivity of oxygen species or avoid their production. Not surprisingly, these same molecules are drivers for evolution. This talk will discuss the problem of oxygen toxicity, the solutions that life evolved, and will highlight lessons from the synchrotron in understanding the importance of intracellular oxidation for space biology and astrobiology.