In the majority of patients with Parkinson's disease (PD), it is now clear that genetic factors contribute to the pathogenesis of PD, although the contribution of genetic and environmental factors remained to be elucidated. Recently, several genes for familial PD (FPD) based on the single gene defects have been mapped and identified. Alpha-synuclein and UCH-L1 are involved in the dominant form of FPD. In contrast, parkin, DJ-1, and PINK1 are responsible for the recessive form of FPD. The presence of different loci or different causative genes indicates that PD is not a single entity but a highly heterogeneous disorder. However, the functions of causative genes may share common pathways such as an ubiquitin-proteasome pathway, oxidative stress, and mitochondrial dysfunction. Thus, the identification and elucidation of the causative genes should enhance our understanding of the pathogenesis of not only FPD, but also sporadic PD.