In this chapter, we summarize the manganese-catalysed transformations developed over the last 20 years. Because manganese is the third most abundant transition metal according to the Clarke number, the price is cheaper than that of almost any other transition metal. Therefore, the development of synthetic organic reactions using manganese catalysts is highly important and desired. In addition, several reactions, which are specific to manganese catalysts, have also been reported. This chapter describes (1) oxidation reactions (epoxidation of alkenes, C-H oxidation, and dehydrogenation); (2) reduction (of alkenes and heterocycles, alkynes, carbonyl compounds, imine derivatives, nitriles and amides, CO2 and CO, and alcohols); (3) carbon-carbon bond formation (addition to unsaturated molecules and cross-coupling reactions); (4) C-H activation (addition to C-C multiple bonds, substitutive C-H functionalization, C-H alkylation, C-H arylation, C-N bond forming reactions, and domino C-H activation reactions); and (5) others (cyclization, annulation, addition to olefins, coupling reaction, C-C bond cleavage, and various others).