Ionic liquids hold potential as green solvents because of their lack of vapour pressure, and are opening up a burgeoningly new field of nonaqueous enzymology. As compared to those observed in conventional organic solvents, enzymes in ionic liquids have presented enhanced activity, stability, and selectivity. Advantages of using ionic liquids over the use of normal organic solvents as reaction medium for biocatalysis also include their high ability of dissolving a wide variety of substrates, especially those highly polar ones, and their widely tunable solvent properties through appropriate modification of the cations and anions. In this review, we discuss the solvent properties of ionic liquids, their effects on enzyme performance such as activity, stability and selectivity, and their applications in biocatalysis. (c) 2005 Elsevier Inc. All rights reserved.