The cause of most cases of Parkinson's disease (PD) is unknown. Researchers have found genes or genetic variants that are associated with risk for the development of PD, but genetic causes are thought to play a role in only a minority of cases. However, accumulating evidence from epidemiologic studies, laboratory basic research, and animal models indicates that exposure to environmental toxins can increase risk of the disease. These substances include pesticides, solvents, metals, and other environmental pollutants. In this episode, Samuel Goldman, MD, MPH, Professor of Occupational and Environmental Medicine and Neurology at the University of California, San Francisco, a Parkinson's Foundation Center of Excellence, reviews what has been learned so far and explains some of the complexities of doing research on environmental causes of PD.