This paper explores the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV; American Psychiatric Association, 1994) definition of Attention Deficit Hyperactivity Disorder) (ADHD). The use of rating scales to diagnose ADHD was evaluated. Rating scales have been used since the 1970s and are highly influential in the detection of ADHD today. We also examined the advantages and disadvantages of using rating scales. Rating scales seem to change the percentage of individuals who meet the criteria for a diagnosis of ADHD based on different age groups, gender, and ethnicity. Finally, the Conners Teacher Rating Scale (CTRS-R) (Conners, Sitarenios, Parker, & Epstein, 1998) was reviewed. The important changes in the revision of the Conners Teacher Rating Scale-Revised ( CTRS-R) were detailed. The use of the CTRS-R is recommended as part of a multi-faceted assessment to diagnose and evaluate treatment procedures for children and adolescents with ADHD.