The Conners 4th Edition–ADHD Index (Conners 4®–ADHD Index) was created to identify the probability that a youth’s score may resemble that of someone who has ADHD by using the optimal subset of items from the Conners 4 in the most efficient manner. This chapter details the development of the Conners 4–ADHD Index (including item selection and score creation), as well as the psychometric properties of the test 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).