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Preclinical Parkinson's disease (PD) can be defined as a state that precedes the diagnosis of PD but without the presence of the characteristic motor features of the disorder. In such a situation, subtle non-motor features may be present or subclinical abnormalities may exist that can be detected only by physiologic testing. Alternatively, lifelong traits such as genetic mutations may predict the onset of PD in the absence of any clinical or physiologic abnormalities. A number of diagnostic technologies are currently available that can detect both preclinical states and traits that will lead to PD in the future. The current challenges are to refine these technologies and to determine how they should be employed so that their use is ethical, practical, and meaningful to at-risk individuals.