Parkinson’s disease (PD) has long been considered to be a sporadic condition, the causative factors of which consist mainly of environmental interactions. For example, much attention has been paid to the contribution of pesticide exposure (DiMonte, Lavasani, Manning-Bog, 2002; Betarbet, Sherer et. al, 2000), rural living (Priyadarshi, Khuder et al., 2001), and industrial metalworking (Jankovic, 2005). Although a family history is the second most important predictor of PD after age, such familial concurrence has often been construed to be the result of shared environmental factors rather than an indicator of heritability. Although twin studies investigating the concordance rates in PD have shown evidence for a heritable aspect in longitudinal studies (Piccini, Burn et al., 1999), no significant relation is seen in cross-sectional studies (Tanner, Ottman et al., 1999; Wirdefeldt, Gatz et al., 2004). Over the past decade, several causative genes have been identified in kindreds demonstrating a Mendelian incidence of disease. Parametric, genome-wide association studies have also revealed multiple susceptibility loci (Pankratz, Nichols et al., 2002; Scott, Nance, et al., 2001). Besides a handful of exceptions, however, these genetic factors exhibit a greatly reduced penetrance and unique molecular pathologies that confound attempts to adequately explain the causes of sporadic PD. While it now appears that the disease has a significant genetic component, the causative factors of sporadic PD may represent a more heterogeneous balance of genetic predisposition and environmental exposure.