This chapter describes the revision goals and development of the Conners Adult ADHD Rating Scales 2nd Edition (CAARS™ 2). Development of the CAARS 2 began in 2015 and comprised three phases: (1) conceptualization, initial planning, and item development; (2) pilot testing; and (3) standardization that included validation studies and finalizing the CAARS 2 forms. This chapter focuses on the development of the full-length CAARS 2. Please see chapters 11 and 12 for information about the development of the CAARS™ 2–Short and the CAARS™ 2–ADHD Index, respectively.