Parkinson's disease (PD) is classically thought of as a movement disorder characterized by tremor, rigidity and postural instability. Nevertheless, there is growing recognition of prominent cognitive impairment in PD and related disorders, which is responsible for substantial disability in these patients. This review will focus on cognitive impairment associated with Lewy body pathology, including PD with dementia (PDD) and dementia with Lewy bodies (DLB). We will review the epidemiology, clinical evaluation, underlying mechanisms and treatment of cognitive impairment in these patients. Despite differences between PDD and DLB, there is clinical, neuropathological and radiological overlap between these disorders, supporting the view that they represent a spectrum of disease. These observations suggest that common targets for diagnosis and treatment of these disorders can be identified.