Dementia with Lewy bodies (DLB) has recently been identified as a separate disease but diagnosis can be difficult, in particular the differentiation from related dementias of Alzheimer's disease (AD) and Parkinson's disease with dementia (PDD). Careful cognitive assessment may aid differential diagnosis between these different types of dementia and can provide theoretical insight into the nature of the underlying impairments. Recent reviews on DLB have primarily dealt with medical issues of clinical diagnostic criteria, pathology, epidemiology and treatment (Ballard, 2004; Barber et al., 2001; Cercy and Bylsma, 1997; Cummings, 2004; Kaufer, 2004; McKeith, 2002; McKeith et al., 2004a; Rampello et al., 2004) and only a few papers have reviewed cognitive impairments in DLB (Collerton et al., 2003; Lambon-Ralph et al., 2001; Simard et al., 2000). The present paper is more specifically targeted to a neuropsychological audience. It provides an up-to-date, detailed and comprehensive review of the available evidence regarding visual and olfactory perception, attention, cognitive fluctuation, frontal-executive functions, working memory, episodic memory, and semantic memory in DLB relative to AD and PDD. In addition, an attempt is made to relate available data to current theoretical frameworks of cognition. Implications for future research and clinical issues such as the problem of differential diagnosis, and the relation between cognitive impairments and clinical features of visual hallucinations and cognitive fluctuation will be discussed.