Attention deficit−hyperactivity disorder (ADHD) has been a common psychiatric diagnosis in both children and adults since the 1980s and 1990s in the United States. But the diagnosis was much less common—even unknown—in other parts of the world. By the end of the twentieth century, this was no longer the case, and ADHD diagnosis and treatment became an increasingly widespread global phenomenon. As the diagnosis was adopted around the world, the definition and treatment of ADHD often changed in the context of different psychiatric professions, medical systems, and cultures.