The aim of this review is to introduce impulsivity as a topic deserving of more varied research than has previously been the case. The author examines the evidence for impulsivity as both a personality trait & a psychiatric symptom, & reviews the findings from a variety of experimental studies. He also investigates the biological basis of impulsivity, with specific reference to its role in the serotonergic neurotransmitter system, & its impact on suicidal behaviours.