Diagnosing ADHD in adolescents requires a baseline knowledge of the diagnostic criteria and presentations, which change over the course of a person’s development. This chapter outlines the sequence and components of a comprehensive diagnostic process for the teen presenting with inattention, hyperactivity, and/or impulsivity. Included is a brief history of the diagnosis, helpful in understanding much of the confusion surrounding it. We review the many challenges unique to making an ADHD diagnosis during the developmental whirlwind of the teenage years, along with strategies for accurate determinations and medical evaluations that may be indicated.