Schools are uniquely situated to provide information relevant to the assessment and treatment of Attention Deficit-Hyperactivity Disorder (ADHD) across a variety of tasks, settings, and observers. The importance of the school setting to the assessment and treatment of ADHD has resulted in the development of numerous measures appropriate for such assessment, but few guidelines for their use. The purpose of this article is to describe school-based assessment procedures and their relationship to ADHD. These procedures include teacher rating measures, direct observations in classrooms and on playgrounds, peer rating and sociometric measures, and permanent product measures such as academic performance. It is recommended that school-based assessment of ADHD involve the concurrent use of multiple measures to assess adequately the wide variety of symptoms associated with this disorder. It is further recommended that assessment serve the purpose of determining treatment efficacy to avoid unwarranted preoccupation with diagnostic issues that are as yet unresolved. The components of a comprehensive school-based assessment of ADHD are described, and their relationship to school-based treatment is discussed.