The value of evidence-based services is now recognized both within clinical communities and by the public at large. Increasingly, neuropsychologists must justify the necessity of often costly and time-consuming neuropsychological assessments in the diagnosis and treatment of common childhood disorders, such as Attention-deficit/Hyperactivity Disorder (ADHD). Published medical guidelines and prominent researchers, however, have argued against the need for formal neuropsychological assessment of ADHD. The present review examines the literature on developmental outcomes in childhood ADHD, with emphasis on the utility of formal neuropsychological assessment among children diagnosed and treated in primary care settings. The review yields three central findings: 1) adherence to published diagnostic guidelines for ADHD is poor among pediatric and primary care physicians; 2) ADHD most often co-exists with other disorders, thus diagnoses made without formal psychometric assessment can be incomplete or incorrect, ultimately increasing treatment costs; and, 3) untreated children with ADHD, and those who have untreated comorbidities, are at greater risk for poor outcomes in social, academic, vocational, and practical settings. The available literature suggests that neuropsychological assessment provides information that can potentially reduce risks for poor outcomes and improve quality of life among children with ADHD. Controlled studies directly examining the impact of neuropsychological assessments in improving outcomes among children with ADHD are needed.