The past decade has seen an increased focus on the developmental trajectory of attention-deficit/hyperactivity disorder (ADHD), with the recognition that ADHD is, for many, a life-long condition akin to many other chronic illnesses. There has been an increase in the extent to which young children, adolescents, and adults receive a diagnosis of ADHD, yet there remain many poorly understood and controversial issues within the scientific community and the lay public. Do ADHD patients of different ages present with similar manifestations of the disorder, and if so, why was this not recognized for so long? Are there alternative clinical presentations among ADHD patients of different ages? What is the nature of comorbidity in ADHD over the course of development, and what are its functional consequences? How can we best measure and define ADHD, differentiating it from normal activity in young children, on the one hand, and other psychiatric disorders in older children and adults on the other? This is a key issue because ADHD has been a controversial diagnostic entity to many nonpsychiatrists because there is no one laboratory task that defines it. Most importantly, how do we understand issues related to risk and resilience in a longitudinal model, and can we identify factors that predict different clinical outcomes or pathways?