Nature utilizes manganese to catalyze a variety of biologically important reactions, such as the dismutation of superoxide radicals, decomposition of hydrogen peroxide and the oxidation of water to dioxygen. Model compounds have been synthesized to mimic the structures and the functions of these manganese containing enzymes. A number of compounds that show catalytic conversion of the same substrates as those of the metalloenzymes are discussed in this paper. Examples of manganese compounds that have been studied as oxidation catalysts both in aqueous and non-aqueous systems are reported. Our research has focussed on catalyzed oxidation reactions in water by manganese complexes containing 1,4,7-triazacyclononane derivatives. We will discuss oxidation catalysis with Mn(tacn) complexes in aqueous media as well as in non-aqueous systems.