Objective: To determine whether childhood attention-deficit/hyperactivity disorder (ADHD) predicts the subsequent emergence of psychiatric disorders independent of disorders already present in childhood, and to translate the evidence into clinical recommendations. Method: Narrative review of meta-analyses, systematic reviews, prospective longitudinal cohorts, population-based register studies, and clinical practice guidelines identified through searches of PubMed/MEDLINE, Embase, PsycINFO, and Web of Science, prioritizing studies with substantial citation impact. Results: Childhood ADHD robustly and independently predicts later substance use disorders, with 2- to 3-fold elevations that survive adjustment for childhood comorbidity. Prospective and genetically informed studies support an association with incident depression and suicidal behavior, though adjusted community models attenuate these effects. Associations with anxiety and antisocial personality disorder are substantially explained by childhood comorbidity in community samples, while clinic cohorts show an elevated risk window for new onsets confined largely to adolescence. Register and meta-analytic data link ADHD to increased risk of psychotic and eating disorders, and genome-wide studies document extensive shared genetic liability across ADHD, depression, schizophrenia, and autism spectrum disorder. Conclusion: Childhood ADHD is associated with the subsequent emergence of psychiatric disorders, but independence from childhood comorbidity varies by outcome domain. Substance use disorders show the strongest evidence of independent emergence; internalizing outcomes are partially confounded by baseline comorbidity. Systematic monitoring for depression, suicidality, and substance use across adolescence, and structured transition to adult services, are warranted for children with ADHD.