Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder with core symptoms that include hyperactivity, impulsiveness, and inattention, and it is the most common psychiatric disorder among children and adolescents. These core symptoms are continuously recognized throughout the day from childhood to adulthood. Furthermore, children with ADHD from childhood to adulthood might also have various comorbid psychiatric disorders. Recently, bipolar disorder and disruptive mood dysregulation disorder, a new clinical issue, have been discussed as comorbid disorders or differential disorders associated with ADHD. Furthermore, comorbid disorders of ADHD are related to quality of life and family burden. Children with ADHD have poorer long-term outcomes than controls with respect to: academic achievement and attainment, occupational rank and job performance, risky sexual practices and early unwanted pregnancies, substance use, relationship difficulties, marital problems, traffic violations, and car accidents. Irritability of children with ADHD has been a key symptom that clinicians and researchers have used to evaluate the developmental condition of children with ADHD. ADHD is sometimes a chronic disorder that occurs over a long period, increasing the family burden of these children (including health-care costs), which will increase with aging for unremitted children with ADHD. Therefore, clinicians should evaluate not only the mental condition of the child but also the family burden. Children with ADHD should be treated during childhood to reduce their clinical symptoms and family burden.