The two most important redox states of iron in the environment are Fe(II) [ferrous iron] and Fe(III) [ferric iron]. Dissolved Fe(II), relatively soluble Fe(II) minerals and poorly soluble Fe(III) minerals are abundant in pH-neutral soils and sediments. Redox transformation of iron leading either to dissolution, transformation or precipitation of iron minerals is used by many microorganisms to produce energy and to grow. Oxidation of dissolved ferrous iron [Fe(II)] at neutral pH can be catalyzed by acidophilic aerobic and neutrophilic microaerophilic, nitrate-reducing and even phototrophic microorganisms. This presentation will summarize the current knowledge and show results regarding mechanisms, physiology, ecology and environmental implications of microbial Fe(II) oxidation. Special focus will be on microaerophilic Fe(II)-oxidizing bacteria that thrive in gradients of ferrous iron and oxygen (e.g. at the surface of rice roots in paddy soil and in surface near lake sediments) and on phototrophic Fe(II)-oxidizing bacteria that were suggested to have contributed to the deposition of ancient rock deposits, so-called Banded Iron Formation, billions of years ago.