This review addresses the most current and widely used methods of assessing childhood and adolescent externalizing disorders. Interviews, rating scales, and self-report instruments are described, and their strengths and weaknesses are discussed. Direct observational techniques in naturalistic and analogue settings are also reviewed. Throughout the article, commentary is offered regarding the psychometric adequacy and clinical validity of these instruments. It is suggested that, although the instruments presently used to assist in diagnosing externalizing disorders generally possess adequate reliability and representational validity, evidence of elaborative validity is lacking. Clinicians and researchers are encouraged to adopt a broader conceptualization of the diagnostic process, to question existing standards for establishing validity, and to consider alternative means of demonstrating diagnostic utility.