The Stop Signal Task is a measure of inhibitory control in which subjects must press a button in response to a stimulus. On certain trials, the subject receives a second stimulus (the Stop Signal) after the primary stimulus and must withhold his/her response during those trials. The onset of the Stop Signal is varied, sometimes coming immediately after the primary stimulus (inhibition is easy); at other times, the Stop Signal arrives quite late, making inhibition difficult. Results from the Stop Signal Task were obtained from children with attention-deficit/hyperactivity disorder (ADHD) and from controls; children with ADHD had significantly more difficulty inhibiting their responses than did controls. In a second study, results from the Stop Signal Task were obtained from a large sample of both behaviorally disturbed and community children; variables from the Stop Signal Task correlated well with both laboratory observations and teacher ratings of inattention and hyperactivity.