Dementia with Lewy bodies (DLB) took many years to crystallize into a recognizable clinico-pathologic entity. Neuropathologic findings of α-synuclein lesions in Lewy bodies and neurites correspond to dysfunction of the substantia nigra, and lesions in subcortical, limbic, and cortical regions are associated with cognitive, neuropsychiatric, and motor manifestations, which may be modified by coexisting Alzheimer tangle pathology. Clinical diagnostic criteria combining cognitive decline with features of parkinsonism, fluctuation of cognition, and visual hallucinations have been revised several times. The latest revision recognizes the importance of REM sleep behavior disorder as a clinical feature and bridges DLB and Parkinson's disease with dementia by removing an arbitrary 1-year separation between onset of motor and cognitive symptoms. Neuropathologic diagnosis now assigns a weight to both α-synuclein and Alzheimer's disease lesions. Treatment remains symptomatic and usually requires sensitive manipulation of a combination of medications to try to stabilize or improve cognitive, behavioral, and motor symptoms while minimizing side effects.