Behavioural symptoms such as anxiety, depression and psychosis are common in Parkinson's disease (PD), and dementia occurs in about 90% of the patients. These symptoms can be more disabling than the motor dysfunction and they negatively impact quality of life, increase caregiver distress and are more frequently associated with nursing home placement. Depression can be treated with counselling and pharmacotherapy. Tricyclic antidepressants or selective serotonin reuptake inhibitors are widely used, but there is still need for controlled clinical trials. Management of psychosis in PD is complex and includes elimination of identifiable risk factors, reduction of polypharmacy and administration of atypical neuroleptics, which can alleviate psychotic symptoms without worsening motor functions. Clozapine is the best documented atypical neuroleptic shown to be effective against psychosis in PD patients. Cholinesterase inhibitors may prove additional benefit in psychotic PD patients. Recent evidence from small double-blind and open-label trials suggests that cholinesterase inhibitors may be effective in the treatment of dementia associated with PD.