This presentation reviews recent advances in the genetics of Attention-Deficit/Hyperactivity Disorder (ADHD) and their direct implications for clinical practice. ADHD is defined as a highly heritable, polygenic disorder arising from the accumulation of thousands of common genetic variants rather than a single gene. The data demonstrate that ADHD represents the extreme end of a continuous dimensional trait in the general population, suggesting that subthreshold symptoms are clinically significant and often require attention. Evolutionary analysis reveals that ADHD risk variants have persisted since Paleolithic times, countering the myth that the disorder is solely a product of modern environments. The presentation details how ADHD shares genetic roots with widespread psychiatric and somatic comorbidities, explaining the complex clinical presentations often seen by practitioners. Furthermore, it distinguishes between causal environmental risks and gene-environment correlations, which often confound perceived risk factors. While the genetic evidence firmly establishes ADHD as a brain-based disorder—helping to reduce stigma—the presentation concludes that current polygenic risk scores and pharmacogenetic tools lack sufficient accuracy for routine clinical diagnosis or treatment selection. Patients and clinicians are encouraged to use this genetic knowledge to understand comorbidity and avoid therapeutic nihilism.