Attention-Deficit/Hyperactivity Disorder is a relatively common condition of childhood onset and is of significant public health concern. Over the past two decades there have been 19 community-based studies offering estimates of prevalence ranging from 2% to 17%. The dramatic differences in these estimates are due to the choice of informant, methods of sampling and data collection, and the diagnostic definition. This article provides a critical review of the community-based studies on the prevalence of ADHD in children and adolescents. Based on the 19 studies reviewed, the best estimate of prevalence is 5% to 10% in school-aged children. The review also examines age and gender effects on the frequency of ADHD. The article closes with a discussion of psychosocial correlates and patterns of comorbidity in ADHD.