It is estimated that up to 80% of patients with Parkinson's disease will eventually develop cognitive impairment over the course of their illness. Even at the time of diagnosis, cognitive impairment has been reported in 20-25% of patients. Commonly affected cognitive domains are executive function, visuospatial ability and attention control. In addition, patients with Parkinson's disease dementia may present with deficits in language function and verbal memory. Psychosis may occur in approximately 40% of patients with Parkinson's disease, and is associated with an increased risk of developing cognitive impairment. Studies have shown that patients with Parkinson's disease with a history of visual hallucinations had an increased risk of developing dementia, four to eight years following diagnosis of the disease. Other clinical risk factors associated with cognitive decline in patients with Parkinson's disease include older age of onset, severe motor symptom burden and in particular akinetic-rigid subtype and olfactory dysfunction. Patients with Parkinson's disease who present with symptoms of cognitive decline, behavioural changes or psychotic symptoms should be referred for further investigation.