The general aspects and recent developments of enzymatic reactions in organic solvents are reviewed as a new technique for utilization of enzymes as synthetic catalysts. Characteristic features of different reaction systems are described in view of the interactions between the reaction components, that is enzyme, substrate, organic solvent, and water. Significant attention is focused on the effects of the nature of organic solvent and water content on activity and specificity of proteases for synthetic and hydrolytic reactions of esters and peptides. Effects of immobilization and complex formation of enzymes with solid materials in organic solvents are also described in terms of activity and stability of the enzymes.