Organophosphates are among the most commonly used and most toxic pesticides. They act directly on the nervous system by inhibiting the neurotransmitter acetylcholine. Organophosphates evoke a consistent pattern of physical symptoms. They also have acute psychological and behavioral effects, such as anxiety, depression, and cognitive impairments. Research suggests that moderate levels of acute poisoning may cause persistent problems. Long-term psychological effects of low-level exposure, however, have not been determined satisfactorily. Some research has documented cognitive and emotional deficits due to chronic exposure to organophosphates, but not all studies have found ill effects. To date, psychologists have played only a small role in studying the psychological effects of organophosphates, despite the substantial contribution their expertise could make.