Abstract: Impulsivity, closely related to the construct of response (dis)inhibition, is central to conceptions of both attention-deficit/hyperactivity (ADHD) and aggressive-spectrum or disruptive behavior disorders. The multifaceted nature of inhibitory deficits requires careful specification in any explanatory accounts of psychopathology. A host of brain regions and neural interconnections are involved in response inhibition; neural models are likely to be complex at the levels of neurotransmitter systems and white-matter tracts. Despite the substantial heritability of ADHD and the substantial continuity of early-onset forms of aggression, developmental processes (including gene-environment correlations and interactions) and transactional models are crucial to the unfolding of regulated versus dysregulated behavioral outcomes. Thus, stressful prenatal and childhood environments must be investigated with as much precision as genetic loci and neural pathways. Differentiating executive inhibition (believed to be largely dopaminergic and frontal/frontal-striatal in nature) from motivational inhibition (believed to be largely noradrenergic/serotonergic and limbic in nature) is necessary to distinguish subtypes of youth with attentional and aggressive problems, and to differentiate key etiologic processes. Indeed, the executive function deficits in children with ADHD appear to independent of their emotion dysregulation, which is specific to an aggressive subgroup. Sex differences in response inhibition and sex differences in its linkages to psychopathology are relatively unexplored. For progress in subsequent research to occur, the following are required: precision in measurement at both biological and behavioral levels; contrasts with clinical comparison samples and comorbid groups (as well as normal control samples); prospective longitudinal investigations; and attention to both developmental processes and contextual variables, including stressful life events, socialization practices, and cultural parameters.