Dementia with Lewy bodies (DLB) and Parkinson's disease-dementia (PDD) are two closely related major neurocognitive disorders with Lewy bodies of unknown etiology, showing notable overlap in their clinical presentation, pathological features, biochemistry, and genetic risk factors. According to international consensus, their diagnosis is based on an arbitrary distinction between the time of onset of motor and cognitive symptoms: dementia preceding parkinsonism in DLB, while it develops after onset of parkinsonism in PDD (the one-year rule). Clinically, both syndromes show cognitive impairment with severe deficits in executive function, visuo-spatial processing, fluctuating attention and parkinsonism, with higher prevalence of Alzheimer-type lesions in DLB that may account for earlier onset and severity of cognitive deficits. These are also associated with multiple neurotransmitter deficits indicating that cognitive impairment in Lewy body disesses is multifactorial. Recent intra vitam neuroimaging, clinico-pathological studies and animal models suggest that DLB and PDD represent closely related but different, heterogenic subtypes of an α-synuclein-associated disease spectrum (Lewy body diseases) or parts of a continuum with PDD at the mild end of the spectrum, DLB in the middle, and DLB+AD at the more severe end. In view of the controversies about the nosology of both disorders, continuous effort is necessary to differentiate them more clearly and to clarify the underlying pathogenic mechanisms in order to enable effective mechanistic-based treatment, while, currently, no disease-modifying therapies are available.