Parkinson's disease (PD) is characterized by its motor impairment. However, non-motor symptoms such as psychiatric disorders, autonomic disturbances and sleep disorders frequently complicate the course of the disease. In particular, psychiatric disturbances including cognitive impairment, depression and psychosis impact these patients considerably. Approximately 31 % of PD patients suffer from cognitive impairment and dementia. Currently, two different clinical presentations are distinguished in PD patients, who present with dementia: Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB), which are two different presentations of a single underlying disease process leading to the deposition of alpha-synuclein. Clinically, PDD is distinguished from DLB alone by the different temporal manifestations of extrapyramidal motor symptoms. Dementia is characterized by a subtle onset and progressive cognitive decline with a predominant dysexecutive syndrome, which can be accompanied by different behavioral symptoms such as hallucinations, depression, anxiety and sleep disorders. Dysregulation of different neurotransmitters has been associated with cognitive decline, but reduced cholinergic transmission is currently thought to be the pivotal mechanism in the development of cognitive dysfunction. Therefore, cholinesterase inhibitors are used in the treatment of dementia and accompanying behavioral symptoms in PDD and DLB. The occurrence of dementia impacts not only the patients themselves but also their care-givers and family.This article focuses on the clinical issues related to both disorders and is based on a meeting of experts which took place in April 2008 in Dresden.