The Conners Adult ADHD Rating Scale 2nd Edition–ADHD Index (CAARS™ 2–ADHD Index) is an empirically-derived scale that effectively differentiates people diagnosed with ADHD from those in the general population. This chapter details the development of the CAARS 2–ADHD Index (including item selection and score creation), as well as the psychometric properties of the index by providing evidence in accordance with test standards (American Educational Research Association [AERA], American Psychological Association [APA], & National Council on Measurement in Education [NCME], 2014).