Studies addressing the relationship between neurotransmitter functioning and violent crime are reviewed. A rich literature exists to support the notion that monoamine (i.e., serotonin, dopamine, and norepinephrine) neurotransmitter functioning is related to human aggressive behaviour. Results from these studies provide, at best, indirect evidence that neurotransmitter abnormalities are involved in violent criminal behavior. Few studies have specifically addressed the role of neurotransmitter functioning in violent crime. To illustrate how current knowledge in this area has been applied in forensic settings, a case study in which neurotransmitter functioning was introduced as evidence to support an insanity defense is presented. Potential problems associated with such defenses are discussed. © 1998 John Wiley & Sons, Ltd.