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The description and discussion of the many aspects of Parkinson disease (PD) is dogged by the fact that PD is an ambiguous concept. What we call PD is an etiologically and clinically heterogeneous metasyndrome overlapping a number of other disorders. Definitions, diagnostic criteria, and classification schema provide essential common vocabularies for communication among clinicians and researchers. Definitions, diagnostic criteria, and classification schemes for PD, however, are constructs imposed on variable and imperfectly understood disease biology. Difficulties with the formulation of PD definitions, diagnostic criteria, and classification schemes include historical and contemporary use of competing definitions of PD, disease heterogeneity, the existence of disorders with overlapping clinical and pathological features, and ambiguous use of the terms “disease” and “syndrome.” For the purposes of exposition and discussion in this volume, the following inclusive—perhaps vague—definition of PD is employed: Parkinson disease is an age-related neurodegenerative syndrome characterized primarily by clinical features of parkinsonism, notably bradykinesia, consequent to relatively selective, early degeneration of dopaminergic nigrostriatal terminals. It is gradually progressive over the course of several years. Its early clinical features are driven by striatal dopaminergic denervation with prominent later clinical features secondary to extrastriatal pathologies.