This chapter briefly presents the most important microbially mediated redox reactions in ecosystems. The chapter introduces redox reactions and their terminology, electron donors and acceptors, and energy yields from redox reactions. It then describes some of the most important microbially mediated redox reactions and the environments in which each typically occurs. The chapter presents the sequence in which redox reactions typically occur as electron acceptors are successively depleted, and briefly discusses why microbially mediated redox reactions are important in ecosystem element cycles.