Behavioral and psychiatric problems associated with idiopathic Parkinson's disease (PD) include cognitive dysfunction, drug-related psychosis, depression, anxiety, apathy, fatigue and sleep disturbance. These nonmotor symptoms are a significant cause of disability at all stages of illness. Cognitive dysfunction spans a continuum from circumscribed cognitive impairments to severe global dementia which can occur in up to 10-30% of advanced PD patients. Psychosis develops in 20-30% of PD patients receiving chronic antiparkinsonian therapy. Visual hallucinations and paranoid delusions are the most frequent symptoms. The gradual elimination of drugs of lesser priority that may affect cognition and/or cloud the sensorium constitutes the first step in the management of cognitive and psychotic symptoms. Atypical neuroleptic agents are an invaluable tool in those cases in which maximum drug regimen simplification is not adequate or results in unacceptable immobility. Depression and anxiety often go unrecognized although they are eminently treatable and may be important contributors to the morbidity of PD. They are present in 30-40% of PD patients and frequently occur together in association with other nonmotor symptoms such as apathy, fatigue and sleep disturbance. A combination of early recognition, counseling, antidepressant therapy, antianxiety and well-balanced antiparkinsonian therapy sets the stage for improved quality of life for patients with PD.