The chapter discusses the cellular defenses against superoxide (O₂⁻) and hydrogen peroxide (H₂O₂) in microorganisms. It highlights the evolution of these defenses, which were developed in the absence of selective pressure to avoid reactivity with oxygen. Microorganisms have evolved mechanisms to protect themselves from partially reduced oxygen species, such as O₂⁻ and H₂O₂, which are more chemically reactive than molecular oxygen. These species are generated as by-products of aerobic metabolism and can damage biomolecules, including enzymes and DNA. The chapter reviews the sources of these oxidants, including redox-cycling compounds excreted by plants and microbes to suppress competitors, and chemical processes involving reduced metals and sulfur species. It also discusses the biomolecules that are damaged by O₂⁻ and H₂O₂, such as dehydratases and DNA, and the mechanisms of damage, including the Fenton reaction. The chapter further explores the inducible responses to reactive oxygen species, such as the SoxR(S) regulon for superoxide stress and the OxyR and PerR regulators for H₂O₂ stress. It details the mechanisms of scavenging systems, including superoxide dismutase (SOD) and catalases, and the exclusion and export of redox-cycling antibiotics. The chapter also covers the repair processes for iron-sulfur proteins and DNA, and the role of methionine sulfoxide reductase and disulfide reducing systems in protein repair. Finally, the chapter discusses the control of unincorporated iron levels and the protective role of manganese against oxidative stress. It concludes by reflecting on the progress made in understanding intracellular oxidative stress and the remaining puzzles that need to be addressed.The chapter discusses the cellular defenses against superoxide (O₂⁻) and hydrogen peroxide (H₂O₂) in microorganisms. It highlights the evolution of these defenses, which were developed in the absence of selective pressure to avoid reactivity with oxygen. Microorganisms have evolved mechanisms to protect themselves from partially reduced oxygen species, such as O₂⁻ and H₂O₂, which are more chemically reactive than molecular oxygen. These species are generated as by-products of aerobic metabolism and can damage biomolecules, including enzymes and DNA. The chapter reviews the sources of these oxidants, including redox-cycling compounds excreted by plants and microbes to suppress competitors, and chemical processes involving reduced metals and sulfur species. It also discusses the biomolecules that are damaged by O₂⁻ and H₂O₂, such as dehydratases and DNA, and the mechanisms of damage, including the Fenton reaction. The chapter further explores the inducible responses to reactive oxygen species, such as the SoxR(S) regulon for superoxide stress and the OxyR and PerR regulators for H₂O₂ stress. It details the mechanisms of scavenging systems, including superoxide dismutase (SOD) and catalases, and the exclusion and export of redox-cycling antibiotics. The chapter also covers the repair processes for iron-sulfur proteins and DNA, and the role of methionine sulfoxide reductase and disulfide reducing systems in protein repair. Finally, the chapter discusses the control of unincorporated iron levels and the protective role of manganese against oxidative stress. It concludes by reflecting on the progress made in understanding intracellular oxidative stress and the remaining puzzles that need to be addressed.