The epidemiologic studies reviewed here have provided insights into the etiology of PD. Evidence increasingly suggests that, like many other chronic age-related diseases, PD is a multifactorial disorder, with both genes and environment contributing to risk. As the elderly population of the world grows, incidence and prevalence of PD will continue to increase, underscoring the importance of further delineating risk factors. The introduction of levodopa and other pharmacologic therapies over the last 2 decades has postponed disease morbidity and mortality, but morbidity and mortality still are increased markedly relative to unaffected individuals. The development of therapies that may slow disease progression makes early identification and treatment of PD particularly important. Investigations of early markers of PD, or markers of disease susceptibility, are critical areas for future research. These efforts all will be aided by careful collaboration between epidemiologists and laboratory scientists.