We are at a critical juncture in our knowledge of the etiology and pathogenesis of Parkinson disease (PD). It is clear that PD is not a single entity simply resulting from a dopaminergic deficit; rather it is most likely caused by a combination of genetic and environmental factors and, although there is extensive new information on the etiology and pathogenesis of PD that may advance its treatment, new syntheses of this information are needed. The first part of this two-part, state-of-the-art review by leaders in Parkinson research critically examines the field to identify where new knowledge and ideas might be helpful for treatment purposes. Topics reviewed in Part I include the definition of the disease, neuropathologic contributions, and epidemiologic, environmental, and demographic issues. © 2007 American Association of Neuropathologists, Inc.