The diagnosis of attention-deficit/hyperactivity disorder (ADHD) in adults can be a challenging process because it includes making judgments based on clinical interviews, rating scale results, informant ratings, and objective supporting evidence. The patient evaluation should gather information on the severity and frequency of symptoms, the establishment of childhood onset of symptoms, the chronicity and pervasiveness of symptoms, and the impact of symptoms on major life activities. Some of the rating scales being used in the adult population are the Conners' Adult ADHD Rating Scales, the Brown Attention-Deficit Disorder Scale for Adults, the Wender Utah Rating Scale, the ADHD Rating Scale and ADHD Rating Scale-IV, the Current Symptoms Scale, and the recently-developed Adult ADHD Self-Report Scale-v1.1 Symptom Checklist. More research is needed to establish the usefulness of self-administered rating scales compared with investigator-administered scales in the assessment and diagnosis of adult ADHD.