ADHD symptoms "run in families". However, relatively little is known about the ways in which parents' symptoms might additively and interactively work with the parenting environment, to influence (and be influenced by) the developmental trajectory of symptoms in children and adolescents. In this commentary on the special section addressing this gap in knowledge, emphasis is placed on the importance of replicating and extending family-wide studies of ADHD symptoms and etiology. The current papers exemplify the leading-edge of such efforts, demonstrating the feasibility and rigor with which studies are being conducted, utilizing longitudinal and experimental designs. Families and parenting environments operate as a system in which individuals influence each other's symptoms and functioning. In so doing, parents produce tremendous variability within (as well as between) each family in individuals' ADHD symptoms from childhood through adulthood, via gene-environment transactions that may even begin during prenatal development.