Over the last few years, several genes for monogenic forms of Parkinson's disease (PD) have been mapped and/or cloned. Mutations have been identified in the gene for alpha-synuclein in rare families with dominant PD, indicating that aggregation of this protein in Lewy bodies is probably a crucial step in the molecular pathogenesis of the disorder. A much more common cause for dominant PD, mutations in the gene for leucine-rich repeat kinase 2 (LRRK2), has recently been identified. Mutations in the parkin gene, in DJ-1 and PINK1 all cause autosomal recessive parkinsonism of early onset. These genes have been implicated in the proteasomal protein degradation pathway, in the oxidative stress response and mitochondrial function. Mutations in recessive genes probably are pathogenic through loss-of-function mechanisms, suggesting that their wildtype products protect dopaminergic cells against a variety of insults. Evidence is emerging that at least some of these genes may play a direct role in the etiology of the common sporadic form of PD. Further, it is likely that the cellular pathways identified in rare monogenic variants of the disease also shed light on the molecular pathogenesis in typical sporadic PD.