1. In children the disorder is characterized by inattentiveness to external direction, impulsive behavior and restlessness. However, children with the inattentive type are actually often sluggish and hypo-active.
  2. In adults the problem is often an inability to structure their lives and plan simple daily tasks. Thus, inattentiveness and restlessness often become secondary problems.

A diagnosis of ADHD is made based on a checklist of symptoms that can be found in DSM-IV-TR. The Centers for Disease Control and Prevention (CDC) emphasizes that a diagnosis of ADHD should only be made by trained health care providers. This is important as many of the criteria can be readily misinterpreted and the prescribed drugs can be very dangerous.

ADHD is considered by some to be a problem all over the industrialized world, although in no other country are children diagnosed with this disorder as often as in the United States.

According to the 2000 edition of DSM-IV-TR, ADHD affects three to seven percent of all children in the U.S. According to 2002 data from the CDC's annual National Health Interview Survey, released in 2004, nearly 4 million children younger than 18 in the United States had been diagnosed with attention deficit hyperactivity disorder (ADHD). The 2002 data indicated that twice as many boys were diagnosed with ADHD as girls (10% vs. 4%). Some experts theorize that ADHD is under-diagnosed in girls, since their symptoms tend to be less dramatic than those in boys and thus draw less attention from parents and teachers.

Psychological testing for ADHD:
Psychological testing for ADHD symptoms generally consists of obtaining multiple types of assessments. These usually include a clinical interview reviewing the DSM-IV criteria for ADHD diagnosis. The interview also needs to rule out as much as possible other types of syndromes which can cause attention problems, such as depression, anxiety, allergies and psychosis. Rating scales can be administered which provide measurement of the person's own view of their symptoms, as well as the views of parents, teachers, and significant others.

Finally, computerized tests of attention can be helpful in providing a further independent assessment. These different assessments may not be consistent, but do provide a view of the person's difficulties. Subjectivity of the analysis can be compounded by the fact that physicians generally need not order psychological testing in order to make the diagnosis of ADHD, but many doctors use this kind of assessment to avoid over-diagnosis and treatment. The process of obtaining referrals for such assessments is being promoted vigorously by the President's New Freedom Commission on Mental Health.

Other forms of testing:
Neurometrics, PET scans, or SPECT scans have been used for a more objective diagnosis. However, these are not usually suitable for very young children.