Today, attention deficit hyperactivity disorder (ADHD) is the most common and most studied psychiatric disorder of childhood, affecting approximately five percent of school-aged children. That means that there are probably at least two children with ADHD in any average elementary school class. In the last 20 years, there has been an explosion in knowledge about ADHD, the primary symptoms of which include excessive impulsiveness, inattention, inconsistency, and inappropriate physical activity. Researchers at Baltimore's Kennedy Krieger Institute have been investigating brain development and behavior in children with this complex disorder for nearly two decades, and this article reviews 11 lessons learned from this research, along with some implications for parents and teachers of children who are affected.