This chapter describes the development of the Conners 4th Edition (Conners 4®). Development of the Conners 4 began in 2015, and involved three phases: (a) conceptualization, initial planning, and item development; (b) the pilot phase; and (c) the standardization phase that included validation studies, creation of the final forms, and the U.S. Spanish and French-Canadian translations. This chapter focuses on the development of the full-length Conners 4. Please see chapters 11 and 12 for information about the development of the Conners 4th Edition Short (Conners 4®–Short) and the Conners 4th Edition ADHD Index (Conners 4®–ADHD Index), respectively.