This article examines evidence-based assessment practices for attention deficit hyperactivity disorder (ADHD). The nature, symptoms, associated features, and comorbidity of ADHD are briefly described, followed by a selective review of the literature on the reliability and validity of ADHD assessment methods. It is concluded that symptom rating scales based on the Diagnostic and Statistical Manual of Mental Disorders (4th ed. [DSM-IV]; American Psychiatric Association, 1994), empirically and rationally derived ADHD rating scales, structured interviews, global impairment measures, and behavioral observations are evidence-based ADHD assessment methods. The most efficient assessment method is obtaining information through parent and teacher rating scales; both parent and teacher ratings are needed for clinical purposes. Brief, non-DSM based rating scales are highly correlated with DSM scales but are much more efficient and just as effective at diagnosing ADHD. No incremental validity or utility is conferred by structured interviews when parent and teacher ratings are utilized. Observational procedures are empirically valid but not practical for clinical use. However, individualized assessments of specific target behaviors approximate observations and have both validity and treatment utility. Measures of impairment that report functioning in key domains (peer, family, school) as well as globally have more treatment utility than nonspecific global measures of impairment. DSM diagnosis per se has not been demonstrated to have treatment utility, so the diagnostic phase of assessment should be completed with minimal time and expense so that resources can be focused on other aspects of assessment, particularly treatment planning. We argue that the main focus of assessment should be on target behavior selection, contextual factors, functional analyses, treatment planning, and outcome monitoring.