Psychosis in Parkinson's disease (PD) is a fairly common and vexing problem. Although it can occur at any stage of the illness, it is a particularly important issue for patients who are in the later stages of PD and have been chronically treated with anti-PD medications. The exact pathophysiology of PD-related psychosis remains a mystery. Neurochemical imbalances, sleep disturbances, and visual processing abnormalities in PD have been implicated in its pathogenesis. Treatment of psychotic symptoms should occur only after potential medical and environmental causes of delirium have been eliminated or addressed. Initial pharmacologic changes should include limiting the patient's anti-PD medications to those that are necessary to preserve motor function. Should that fail, an atypical antipsychotic agent is presently the treatment of choice. An emerging treatment option is the use of acetylcholinesterase inhibitors. This article reviews what is known about the epidemiology, risk factors, pathophysiology, and treatment of PD-related psychosis.