The primary purpose of the Conners Adult ADHD Rating Scales 2nd Edition (CAARS™ 2) is to assess core symptoms and associated features of Attention-Deficit/Hyperactivity Disorder (ADHD) in adults. This chapter provides background relevant to that purpose, including a review of the key features, diagnostic criteria, and epidemiology of ADHD in adults. It then describes the components of a comprehensive adult ADHD assessment and concludes with an overview of the CAARS 2.