Cognitive impairment and dementia are significant sequelae of Parkinson disease (PD) and comprise a key feature of dementia with Lewy bodies (DLB), a disease with similar clinical and neuropathological features. Multiple independent causes have been implicated in PD dementia (PDD) and DLB, among them the accumulation of β-amyloid, a neuropathological hallmark of Alzheimer disease. Over the last decade, PET imaging has emerged as a viable method to measure amyloid burden in the human brain and relate it to neurodegenerative diseases. This article reviews what amyloid imaging has taught us about PDD and DLB. Current data suggest that brain amyloid deposition tends to be more marked in DLB, yet contributes to cognitive impairment in both DLB and PD. These results are broadly consistent with neuropathology and CSF studies. β-Amyloid may interact synergistically with other pathological processes in PD and DLB to contribute to cognitive impairment.