In the past few years, the genetic contribution to Parkinson's disease (PD) has gained major attention and has resulted in the identification of the first mutant gene, called alpha-synuclein, involved in the pathogenesis of autosomal-dominant PD. alpha-Synuclein is a major component of Lewy bodies, which are a neuropathological feature of PD. Furthermore, deletions in the parkin gene have been identified as the primary cause in rare forms of autosomal-recessive juvenile PD. The elucidation of polygenic changes in the dopamine pathway, mitochondrial dysfunction, and metabolism of xenobiotics is now technically possible by means of association and genotype studies. The increasing knowledge of the pathogenesis of PD at a molecular level will have important implications for the development of individual therapeutic strategies to prevent disease progression.