This chapter provides an overview of the most frequent causes of sleep complaints in patients with attention-deficit/hyperactivity disorder (ADHD), discussing the most relevant data from the scientific literature. Clinical experience suggests that stimulants are the first-line pharmacological treatment of ADHD, may negatively impact on sleep, but the effect varies among patients. Sleep in children with ADHD has been investigated in subjective and objective studies. The former have used sleep questionnaires filled out by the parents or, less frequently, by the children/adolescents themselves. In recent years, there have been significant improvements in study methodology in research examining sleep in ADHD, but given the still limited amount of data, it is clear that the field needs more methodologically sound studies. As for studies investigating subjective measures of sleep, most of these used parental reports. However, older children, and in particular adolescents, can provide information on sleep difficulties not detected by parents.