The inability to exert appropriate inhibitory control over one?s behavior is an important symptom of many psychiatric illnesses. The stop signal paradigm, which involves withholding a prepotent motor response to a signal, has proved useful in assessing this deficit in children with a range of disorders, particularly attention deficit hyperactivity disorder (ADHD). In order to adapt the paradigm for use in fMRI studies, it is important to address two methodological issues: 1) the substitution of a visual stop signal for the more commonly-employed auditory stop signal, and 2) the relative merits of the traditional stop task (subjects withhold a motor response to the ?stop? signal) vs. the stop-change task (subjects execute alternative motor responses to the ?stop? and ?go? signals). The first aim of this protocol is to compare behavioral data from these alternative stop signal paradigms in control adults, control children, and children with ADHD. These data will be used to test the hypotheses that, on each of these paradigms, children with ADHD will show an inhibitory deficit compared to control children, and control children will show an inhibitory deficit compared to control adults. Collaborations are being formed with Children?s National Medical Center to test the behavioral hypotheses concerning control children and those with ADHD. In addition, behavioral testing is being conducted in children with bipolar disorder, in order to facilitate study of their deficits in impulse control. Preliminary data from bipolar children indicates that they do have a lengthened stop signal response time, indicating deficits in motor inhibition. The second aim of the protocol is to conduct a pilot fMRI study of the stop and stop-change tasks in control adults and adolescents. The study has an event-related design and will test the hypotheses that ventral prefrontal activation will be greater during successful than unsuccessful stop (or stop-change) trials, and that stop and stop-change trials will be associated with more anterior cingulate activation than will go trials. Developmental differences between adults and adolescents will be explored. To date, approximately 20 control adults have been scanned using the paradigm, demonstrating feasibility. These data are being analyzed and recruitment of adolescents and a second group of control adults has begun. The third aim of the study is to develop a version of the stop task with a parametric design and one with behavioral contingencies.