Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous behavioral disorder of uncertain etiology that is always evident first in childhood. The correct diagnosis of the ADHD patient thus requires familiarity with how the diagnosis should be established across the life span. This article provides a description of the DSM-IV syndrome of ADHD, information on prevalence, and an overview of standard methods used in office practice to diagnose ADHD in children. These practice parameters will be examined in the light of the practitioner's current concerns about the validity of the diagnosis.