Parkinson's disease (PD) is the second commonest neurodegenerative disease, exceeded only by Alzheimer's disease (AD). It is estimated that approximately 1 million persons in the United States and 5 million persons in the world suffer from this disorder. PD affects men and women of all races, all occupations, and all countries. The mean age of onset is about 60 years, but cases can be seen in patients in their 20s, and even younger. The frequency of PD increases with aging, and based on projected population demographics, it is estimated that the prevalence will dramatically increase in future decades.