Epidemiologic data on attention deficit hyperactivity disorder (ADHD) are essential for planning health services and implementing strategies of detection and early intervention, with possible substantial benefits on public health. This article addresses methodological aspects of prevalence studies, recent findings on the prevalence of ADHD in childhood and adolescence based on a systematic review, current findings on the persistence of the disorder over time and prevalence in adulthood, and factors associated with ADHD. Evidence from the reviewed literature indicates the importance of methodological aspects in the understanding of epidemiologic findings and the necessity of large-scale cross-national studies. Moreover, governments clearly must direct attention to childhood mental disorders to guarantee a healthy future for their countries.