The role of redox processes in determining the chemistry of iron and manganese is considered systematically. Both metals have soluble reduced forms and insoluble oxyhydroxides which are readily interconverted in the vicinity of a redox boundary. Although the oxyhydroxides are dominant in well-oxygenated waters, measureable concentrations of Fe(II) and Mn(II) can be observed, especially where photochemical reduction occurs. Differences in behaviour are largely attributed to Fe(II) oxidizing more rapidly than Mn(II) and the relative case of reduction of manganese oxyhydroxides. Mechanisms of reduction and oxidation, including the role of micro-organisms, are considered. Particulate and colloidal forms, which are the products of oxidation, are seen to have diverse morphometries and compositions and are as yet poorly characterized. They appear to play a smaller role in the transport of trace metals and phosphorus than was once believed. Various lacustrine environments, sediments, the sediment-water interface and anoxic and oxygenated waters, are considered within a unifying context of the processes occurring at a redox boundary. Markedly elevated concentrations of Fe(II) and Mn(II) in anoxic bottom waters have encouraged detailed field studies of their production, transport and oxidation. Results from diverse sites are brought together to provide an overall understanding of the redox cycling of these two important elements in lakes.