Probably the three most important components to a comprehensive evaluation of patients with attention-deficit hyperactivity disorder (ADHD) are the clinical interview, the medical examination, and the completion and scoring or behavior rating scales. Teachers and other school personnel are often the first to recognize that a child or adolescent might have ADHD, and often play an important role in the help-seeking/referral process. A diagnostic evaluation for ADHD should include questions about ADHD symptoms, other problems including alcohol and drug use, family history of ADHD, prior evaluation and treatment for ADHD. Screening interview or rating scales as well as interviews should be used. When it is feasible, clinicians may wish to supplement these components of the evaluation with objective assessments of the ADHD symptoms, such as psychological tests. These tests are not essential to reaching a diagnosis, however, or to treatment planning, but they may yield further information about the presence and severity of cognitive impairments that could be associated with some cases of ADHD. Screening for intellectual ability and academic achievement skills is also important in determining the presence of comorbid developmental delay or loaming disabilities. The number and type of symptoms required for a diagnosis of ADHD vary depending on the specific subtype. To receive a diagnosis of ADHD, the person must be experiencing significant distress or impairment in daily functioning, and must not meet criteria for other mental disorders which might better account for the observed symptoms such as mental retardation, autism or other pervasive developmental disorders, mood disorders, anxiety disorders. This report aims to suggest a practice guideline of assessment and diagnosis for children and adolescents with ADHD in Korea.