The overarching goal of this prospective cohort study is to test the hypotheses that greater severity of hyposmia is associated with increased risk of cognitive decline in PD and that worsening hyposmia parallels progressive cholinergic limbic denervation. To achieve the goals of this project, patients with PD without dementia or at-risk of dementia or with dementia will undergo longitudinal olfactory, cognitive and clinical testing for 2-4 years. AChE [11C]PMP or VAchT (vesicular acetylcholine transporter) [F18]-FEOBV PET will be performed both at study entry and at 2-years (± 6 months) follow-up. Brain MRI scans will also be performed at study entry and at 2-years (± 6 months) follow-up. Brain Beta-amyloid PET will be performed at study entry or at 2 years (± 6 months). Annual olfactory testing will be performed to better understand dynamic changes underlying the clinical and PET outcome measures.