The PαSS U19 seeks to understand and characterize the preclinical/prodromal phase of α-synucleinopathies. The study will recruit a cohort of iRBD subjects and follow them if/until they convert to PD, LBD, or MSA, using standardized psychometric tests, clinical/neurological evaluations, and diagnostic procedures. PαSS will identify changes associated with the earliest manifestations of α-synucleinopathy disease, including biomarkers of disease progression and potential surrogate endpoints. Biomarkers of α-synucleinopathy burden may range from neurophysiology, to changes in biofluid/tissue, to neuroimaging changes. The eventual goal is to find neuroprotective interventions for α-synucleinopathies.