Attention deficit hyperactivity disorder (ADHD) is a behavioral disorder that affects a person’s ability to pay attention, concentrate, and behave in a way that is appropriate for their age. People with ADHD often have trouble staying organized and being on time, have difficulty finishing tasks, and sometimes behave without thinking about the consequences of their actions. Most people think of ADHD as a childhood disorder, but it also affects millions of adults in the US. The exact cause of ADHD is unknown, but researchers think it may be caused by a combination of genetic and environmental factors.