All Participants: Age 60+. Sufficient English to carry out cognitive testing. Controls: MMSE greater than or equal to 26. Lewy Body/AD Dementia: MMSE greater than or equal to 12. Meet criteria for probable LBD/AD. If taking anti-cholinesterase drugs or memantine, stable for at least 3 months. Presence of reliable informant sufficient to provide information for informant rated scales.Concurrent major psychiatric illness (e.g. major depression). Severe physical illness or comorbidity that may limit ability to fully participate in study. Past history of excess alcohol intake. Past history of neurological illness including stroke. Contra-indications for MR or PET. Psychotropic medications which may significantly interfere with cognitive testing (anti- dementia drugs not an exclusion criteria). Controls: memory complaints or signs/ symptoms of dementia. Past history of Parkinson's disease. Psychotropic medications which may significantly interfere with cognitive testing (anti- dementia drugs not an exclusion criteria). Controls: memory complaints or signs/ symptoms of dementia. Past history of Parkinson's disease. Alzheimer's patients: Past history of Parkinson's disease. All Participants: Contra-indications for MR or PET imaging.Past history of excessive alcohol intake. Past history of other neurological illness including, but not limited to stroke, intracerebral pathology. Psychotropic and other medications which may significantly interfere with cognitive testing (including but not limited to sedative antidepressants, benzodiazepines except low when used as hypnotics, centrally acting anticholinergic drugs). Use of anti dementia drugs (eg anti-cholinesterase drugs or memantine) is not an exclusion criterion. A relevant history of severe drug allergy or hypersensitivity. Have ever participated in an experimental study with an amyloid targeting agent (e.g. anti-amyloid immunotherapy, or γ-secretase inhibitor) unless it can be documented that the subject received only placebo during the course of the trial. Receiving any investigational medications, or participation in a trial with investigational medications within the last 30 days. A radiopharmaceutical imaging or treatment procedure within 7 days prior to the study imaging session.