Ionic liquids have been suggested as potential "green solvents" due to their unique properties such as nonvolatility, nonflammability, and a wide temperature range for liquid phase. This review describes recent advances of biocatalyst reactions in ionic liquids. Enzyme-catalyzed reactions in ionic liquids-transesterification, synthesis, conversion, ammoniolysis, hydrolysis, epoxidation, resolution, and oxidation are presented. The use of ionic liquids for protein folding/refolding and the toxicity of ionic liquids are also discussed.