Summary: Although first suggested by de Duve in 1964, enzyme replacement therapy (ERT) for lysosomal storage diseases did not become a reality until the early 1990s when its safety and effectiveness were demonstrated in type 1 Gaucher disease. Today, ERT is a reality for Gaucher disease, Fabry disease and mucopolysaccharidosis type I (MPS I), and clinical trials with recombinant human enzymes are ongoing in Pompe disease, MPS II and MPS VI, and are about to begin in Neimann–Pick B disease. In addition to ERT, enzyme enhancement therapy (EET) offers a novel therapeutic strategy to increase the residual function of mutant proteins. EET employs small molecules as 'pharmacological chaperones' to rescue misfolded and/or unstable mutant enzymes or proteins that have residual function. EET also offers the possibility of treating neurodegenerative lysosomal disorders since these small therapeutic molecules may cross the blood–brain barrier. The current status of ERT and the prospects for EET for lysosomal storage diseases are reviewed.