Although historically conceptualized as a disorder that was limited to males during middle childhood, ADHD is currently conceptualized as a chronic disorder that persists into adolescence and adulthood for both sexes. Nonetheless, the veracity of adult ADHD continues to be the source of debate. In order to frame this debate, research leading to the conceptualization of ADHD as a chronic disorder is reviewed. A distinction is made between the developmental outcomes versus the developmental course of ADHD. It is concluded that although childhood ADHD is associated with negative developmental outcomes in adolescence and adulthood, questions about the developmental course of ADHD remain. Although it appears that ADHD diminishes with advancing age, a number of methodological limitations prohibit firm conclusions. Recommendations for future studies are made with an emphasis on 1) overcoming extant methodological limitations in the literature and 2) the need for theoretically derived hypotheses regarding continuity and change in ADHD symptomatology over time.