Epidemiological research has confirmed that Parkinson's disease (PD) is found throughout the world and increases exponentially with age. Few good-quality data on the temporal incidence of PD are available, although both mortality and incidence data suggest that the disease may be less common today in younger age groups. Differences in prevalence between identical ethnic groups in different countries support the role of an environmental factor. Any postulated factor must be found commonly in developed countries, among which there appears to be little difference in incidence or prevalence rates. A wide variety of aetiological agents have been considered from infectious, toxic and other exposures. The most robust finding is that non-smokers have a greater risk of disease, although the reason for this is unclear and may relate to differences in pre-morbid personality. Pesticides and head injuries also show consistently elevated risk but are prone to biased measurement. Dietary anti-oxidants require further evaluation. Future research needs to improve on current limited methods of exposure measurement and to attempt more novel designs to overcome bias. More attention should be made on examining what factors determine prognosis and using epidemiological and qualitative methods to determine the needs of patients with PD.