Many authors have contributed to the description of attention deficit/hyperactivity disorder (ADHD) for the two last centuries. In this chapter, we review the current diagnostic criteria, epidemiology, and history of ADHD. The different phenotypes (predominantly inattentive, predominantly hyperactive/impulsive, or combined) and diagnostic process are detailed. The DSM-5 includes the three phenotypes that begin before age 12, are present in at least two settings, and cannot be explained by another condition. Theoretical underpinnings and biological and environmental etiologies reported in the latest literature are discussed. There are many comorbidities associated with ADHD, which are associated with an increase in the negative impact on everyday life. Treatment decisions involve a complex interaction between child's age, symptom severity levels, comorbidities, functional impairments, and parents’ preferences. Medication (psychostimulant and nonstimulant) and psychosocial (mainly behavioral parent training) treatments as well as school-based interventions are described.