CONTEXT: The genetic and environmental link between attention-deficit/hyperactivity disorder in childhood and the adult manifestation of the disorder is poorly understood because of a lack of longitudinal studies with cross-informant data. OBJECTIVE: To explore the relative contribution of genetic and environmental influences on symptoms of attention problems from childhood to early adulthood. DESIGN: Analysis was conducted using longitudinal structural equation modeling with multiple informants. SETTING: The Swedish Twin Study of Child and Adolescent Development. PARTICIPANTS: One thousand four hundred eighty twin pairs were prospectively followed up from childhood to young adulthood. MAIN OUTCOME MEASURES: Symptoms were obtained using parent and self-ratings of the Attention Problems Scale at ages 8 to 9, 13 to 14, 16 to 17, and 19 to 20 years. RESULTS: The best-fitting model revealed high heritability of attention problems as indexed by parent and self-ratings from childhood to early adulthood (h² = 0.77-0.82). Genetic effects operating at age 8 to 9 years continued, explaining 41%, 34%, and 24% of the total variance at ages 13 to 14, 16 to 17, and 19 to 20 years. Moreover, new sets of genetic risk factors emerged at ages 13 to 14, 16 to 17, and 19 to 20 years. CONCLUSIONS: The shared view of self- and informant-rated attention problems is highly heritable in childhood, adolescence, and early adulthood, suggesting that the previous reports of low heritability for attention-deficit/hyperactivity disorder in adults are best explained by rater effects. Both genetic stability and genetic innovation were present throughout this developmental stage, suggesting that attention problems are a developmentally complex phenotype characterized by both continuity and change across the life span.