Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB) are characterized by progressive cognitive decline that involves visuospatial, attentional, and executive dysfunctions; psychiatric disturbances including visual hallucinations, anxiety, apathy, or depression; and a parkinsonian motor syndrome. In PDD, the motor syndrome is established before prominent cognitive decline, whereas in DLB they co-occur or the dementia precedes the motor syndrome. The pathobiology of DLB and PDD is complex but both involve abnormalities of the protein alpha-synuclein. In both conditions, alpha-synuclein aggregates to form Lewy bodies and Lewy neurites. The regional distribution of these inclusions influences the predominance of motor versus cognitive and psychiatric symptoms and signs. As a complicating factor, both vascular and Alzheimer's (AD) pathology are present in many DLB and PDD patients, and the greater the burden of tau pathology, the more clinical features overlap with those of AD. It is still a matter of debate whether the clinical and pathological relationship between DLB and PDD is better characterized as a continuous spectrum rather than dichotomous entities. We review DLB and PDD with an emphasis on clinical features and diagnosis.