With onset often occurring before 6 years of age, attention-deficit/hyperactivity disorder (ADHD) involves attention problems, impulsivity, overactivity, and sometimes disruptive behavior. Impairment usually persists into adulthood, with an estimated worldwide prevalence in adults of 2.5%. Existing gaps in evidence concerning ADHD hinder decision-making about treatment. This article describes and prioritizes future research needs for ADHD in three areas: treatment effectiveness for at-risk preschoolers; long-term treatment effectiveness; and variability in prevalence, diagnosis, and treatment.Using a recent systematic review concerning ADHD completed by a different evidence-based practice center as a foundation, we worked with a diverse group of 12 stakeholders, who represented researchers, funders, healthcare providers, patients, and families, to identify and prioritize research needs. From an initial list of 29 evidence gaps, we enumerated 8 high-priority research needs: a) accurate, brief standardized diagnosis and assessment; b) comparative effectiveness and safety of pharmacologic treatments for children under 6 years of age; c) comparative effectiveness of different combinations of psychosocial and pharmacologic treatments for children under 6 years of age; d) case identification and measurement of prevalence and outcomes; e) comparative effectiveness of psychosocial treatment alone versus pharmacologic and combination treatments for children under 6 years of age; f) comparative long-term treatment effectiveness for people 6 years of age and older; g) relative efficacy of specific psychosocial program components for children under 6 years of age; and h) identification of person-level effect modifiers for people 6 years of age and older. In this article, we describe these future research needs in detail and discuss study designs that could be used to address them.