The NHS describes ADHD (attention deficit hyperactivity disorder) as a condition that affects people's behaviour, and they "can seem restless, may have trouble concentrating and may act on impulse".
Its symptoms can be noticed young, and most cases are diagnosed in children aged under 12.
However, the NHS says sometimes it is diagnosed in adults, although symptoms may improve as people get older, and there can be extra problems, such as sleep and anxiety disorders.
Adults are advised to speak to a GP if they think they have ADHD and have not been diagnosed. Concerns about children should be discussed with their teacher, school or GP if their behaviour is thought to be different to others of their age.
In children the condition can be managed with educational support, advice for parents, and possibly medicine.
In adults, the NHS says medicine is often the first treatment offered, and psychological therapy like cognitive behavioural therapy (CBT) can help.