A review is given of certain aspects of manganese-containing enzymes, such as manganese superoxide dismutase, manganese catalase, and the photosynthetic oxygen-evolving catalyst. A variety of manganese complexes have been synthesized to model either the structure of the active site of these enzymes (structural models), or their function (functional models). Most of the enzymatic reactions can be mimicked by functional models, albeit at much lower reaction rates. Additionally, a number of manganese complexes based on the structural mimics, have been studied as catalysts in oxidation reactions other than the enzymatic ones. Examples of functional models for superoxide dismutase, catalase and the water oxidation catalyst in plants, as well as a number of manganese complexes catalyzing a variety of other oxidation reactions, are also discussed in this review.