Using a cost of illness (COI) framework, this article examines the economic impact of attention-deficit/hyperactivity disorder (ADHD) in childhood and adolescence. Our review of published literature identified 13 studies, most conducted on existing databases by using diagnostic and medical procedure codes and focused on health care costs. Two were longitudinal studies of identified children with ADHD followed into adolescence. Costs were examined for ADHD treatment-related and other health care costs (all but 1 study addressed some aspect of health care), education (special education, 2 studies; disciplinary costs: 1 study), parental work loss (2 studies), and juvenile justice (2 studies). Based on this small and as yet incomplete evidence base, we estimated annual COI of ADHD in children and adolescents at $14,576 per individual (2005 dollars). Given the variability of estimates across studies on which that number is based, a reasonable range is between $12,005 and $17,458 per individual. Using a prevalence rate of 5%, a conservative estimate of the annual societal COI for ADHD in childhood and adolescence is $42.5 billion, with a range between $36 billion and $52.4 billion. Estimates are preliminary because the literature is incomplete; many potential costs have not been assessed in extant studies. Limitations of the review and suggestions for future research on COI of ADHD are provided.