Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by rigidity, bradykinesia, postural instability, and tremor. Clinical decline reflects ongoing degeneration of dopamine-containing neurons. A critical unmet need for clinical research is to improve early detection of these diseases by developing tools to assist with earlier diagnosis. Biomarkers are broadly defined as characteristics that are objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention (Biomarkers Defintions Working Group 2001). Development of reliable biomarkers for PD would dramatically accelerate research on PD etiology, pathophysiology, disease progression and therapeutics. Specific biomarkers may be useful at the onset of neurodegeneration, the onset of disease, and/or to mark disease progression. The biomarkers in this study include brain imaging with a radioactively labelled drug (Beta-CIT), computerized testing of memory, attention, motor speed, judgment and handwriting, and assessments of speech and smell. Subjects may also be asked to provide a blood sample for genetic and biochemical testing.