Attention-Deficit/Hyperactivity Disorder (ADHD) is a mental health condition characterized by difficulty paying attention, impulsive behavior, and hyperactivity. Adult ADHD can lead to problems at work, poor school performance, low self-esteem, and relationship problems. Though ADHD isn't diagnosed in some individuals until adulthood, ADHD symptoms always begin in childhood. The symptoms of adult ADHD may not be as clear as they are in children. For instance, in adults, hyperactivity may decrease while problems with attention, impulsiveness, and restlessness may persist.