The attention deficit hyperactivity disorder (ADHD) is an heterogeneous, complex and common childhood disorder that causes significant impairment of child's functioning. This paper reviews the epidemiology, etiology, diagnosis, comorbidity, course and treatment of ADHD. Special attention is paid to sleep problems both because these 2 conditions frequently co-exist and because there is substantial evidence that ADHD psychopathology and sleep-wake regulation share common neurobiologic mechanisms. Moreover, there could be an overlap between symptoms of ADHD and certain sleep problems such as obstructive sleep apnea syndrome, restless leg syndrome and periodic limb movements of sleep. Children undergoing evaluation for ADHD should be systematically assessed for sleep disturbances because treatment of sleep disorders is often associated with improved symptomatology and decreased need for stimulants.