Parkinson's disease dementia (PDD) ultimately develops in about 80% of patients with Parkinson's disease (PD), and cross-sectional studies have found that some 30% of these patients will experience neuropsychiatric symptoms, such as visual hallucinations and psychosis. The most consistently reported risk factors for dementia in PD are age, severe parkinsonism and mild cognitive impairment. In PDD, both subcortical cognitive and cortical cognitive profiles are described. Specific disorders of sleep, such as rapid eye movement sleep behaviour disorder, excessive daytime sleepiness and sleep attacks, occur frequently. Alzheimer and Lewy body pathology coexist, but the Lewy body pathology in limbic and cortical areas seems to be the main cause of dementia. Neurochemical changes in the biogenic amines and acetylcholine are common, and magnetic resonance imaging studies have shown cortical atrophy in wide cortical areas, including the hippocampus. All PD patients should be screened for mild cognitive impairment and dementia. A large randomised clinical trial showed that the cholinesterase inhibitor rivastigmine has desirable effects on cognition and neuropsychiatric symptoms in PDD patients. Atypical antipsychotic agents may improve psychosis in PDD, but the evidence for this is poor and adverse effects from such therapy are common and may be severe. Non-pharmacological interventions can also be effective but require further study.