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. 1. ADHD is often associated with "executive dysfunction." For many years, the accepted belief was that ADHD just affected attention and activity levels, and that many affected children would outgrow their symptoms as they matured. We now know that ADHD has lifelong effects and that more than attention is involved. A good deal of the recent research on ADHD has shown that it is associated with difficulties in "executive control." Use of the term executive function is now commonplace among researchers, clinicians, and classroom teachers, yet definitions vary (as do the characteristics of the children with ADHD themselves). In general, executive function refers to a group of self-regulatory processes that allow individuals to pick the best strategy needed to "get the job done," based upon all the knowledge and skills a person has acquired, and to actually implement that strategy effectively. To illustrate, consider what goes into baking a cake. You can't begin until you have the raw materials at hand, and these include the individual ingredients, kitchen utensils, an oven, etc. Then you need a strategy for using these raw materials, and this is where executive processes come into the picture. In the case of baking a cake, steps to take would include selection of the right recipe from the countless alternatives available in memory or cookbooks and its step-by-step implementation. Ingredients have to be measured and combined, pans have to be prepared, the oven needs to be preheated to the right temperature, the batter needs to be mixed and baked for just the right time, etc. All this activity relies on executive skills that include initiation, planning, organization, shifting of thought or attention, inhibition of distracting thoughts, and sustained and sequenced behavior. It also involves "working memory," which refers to the ability to hold information actively in mind while performing tasks.