Parkinson's disease is among the most common movement disorders. Its prevalence is roughly 120 in 100,000 in the United States and in other parts of the industrialized world where it has been surveyed. Onset of Parkinson's disease is unusual before age 40. Thereafter, the incidence increases sharply with age, ultimately affecting 1.5% of the population over 65, and 2.5% over 85. The illness was first described as a "shaking palsy" by James Parkinson in the early nineteenth century. There has been an explosion of knowledge since the early 1960s, with improvements in therapeutics that have fundamentally altered the natural history of the disease.