Parkinsonian dementia syndromes entail a combination of motor and cognitive symptoms and a variety of underlying etiologies. Lewy body dementias are most common, encompassing Parkinson disease (PD) with dementia and dementia with Lewy bodies, which share the common pathologic substrate of intracellular neuronal inclusion bodies that contain α-synuclein. Multiple system atrophies (MSAs), which are now divided into parkinsonian and cerebellar subtypes, are related disorders with core features that include autonomic and parkinsonian motor signs and α-synuclein-containing glial intracytoplasmic inclusion bodies. Progressive supranuclear palsy and corticobasal degeneration are parkinsonian dementias that superficially resemble PD and MSA in terms of motor features, but are distinguished pathologically by neurofibrillary tau protein abnormalities. Some other causes of dementia associated with parkinsonism include drug-induced parkinsonism, vascular parkinsonism, normal pressure hydrocephalus, prion diseases including Gerstmann-Sträussler-Scheinker syndrome (see the chapter "Rapidly Progressing Dementias"), Alzheimer disease with extrapyramidal signs (see the chapter "Alzheimer Disease Update"), and metabolic derangements that have a predilection for basal ganglia structures. This review will discuss clinical presentations, differential diagnoses, laboratory and neuroimaging characteristics, and therapeutic strategies for the synucleinopathies and several parkinsonian dementia syndromes.