Assessing impairment is an explicit component of current psychiatric diagnostic systems. A brief parent and teacher rating scale for assessing impairment was developed and studied using attention deficit hyperactivity disorder (ADHD) as an exemplar disorder. The psychometric properties of the Impairment Rating Scale (IRS) were measured in 4 samples. Two included ADHD and matched comparison children and the other 2 a school sample. Overall, IRS ratings exhibited very good temporal stability. They correlated with other impairment ratings and behavioral measures and displayed evidence of convergent and discriminant validity. The IRS was highly effective in discriminating between children with and without ADHD. Evidence that the parent and teacher IRS accounted for unique variance beyond ratings of ADHD symptoms is also presented. The scale is brief, practical, and in the public domain. The results of the studies and implications for the assessment of impairment are discussed.