There is convincing evidence that the Parkinson disease neurodegenerative process begins many years before the onset of motor manifestations. Initial estimates based on nigral neuropathological findings or striatal dopamine imaging suggested a 5- to 6-year preclinical period. However, more recent evidence of Lewy body pathology in other neuronal populations preceding nigral involvement suggests that the preclinical phase may be much longer. Epidemiologic studies of nonmotor manifestations, such as constipation, anxiety disorders, rapid eye movement sleep behavior disorder (RBD), and anemia, suggest that the preclinical period extends at least 20 years before the motor manifestations. Olfactory impairment and depression may also precede the onset of motor manifestations; however, the lag time may be shorter. Recognition of a nonmotor preclinical phase spanning 20 or more years should guide the search for predictive biomarkers and the identification of risk or protective factors for Parkinson disease.