The ubiquitin proteasome system is an important cellular pathway that ubiquitinates damaged proteins and degrades them via the 26S proteasome. Abnormalities of this pathway can result in molecular protein aggregation and have been associated with Parkinson's disease (PD). UCHL-1, an enzyme central to the system, possesses catalytic hydrolase activity that can hydrolyze peptide-ubiquitin bonds and recycle ubiquitin monomers for re-use in the same process. Recently, UCHL-1 has been shown to possess a second dimerisation-dependent ligase activity and, at least in vitro, this ligase activity promotes alpha synuclein aggregation. UCHL-1 was first implicated in PD by the discovery of an I93M mutation identified in a German sib-pair with probable autosomal dominant PD. Although no further UCHL-1 mutations have been identified, a common non-synonymous S18Y polymorphism has been suggested to reduce disease susceptibility in non-mendelian forms of PD. In vitro functional data support this protective effect, with evidence that S18Y possesses reduced ligase activity compared with wild type UCHL-1. One study has found increased hydrolase activity associated with S18Y, although another study has not. Important issues regarding UCHL-1 and its role in PD remain inconclusive, especially regarding the pathogenicity of the mendelian I93M mutation. This review tries to address some of these uncertainties.