The goal of any assessment process is to provide a consistent and accurate determination as to whether an individual has a drug problem. A logical first step is to define a drug problem and then develop instruments that can reliably and validly measure the existence of the problem. Given a set of instruments, researchers need criteria by which to compare them and determine which ones may work best in a particular population or setting. There is no single generally accepted definition or measure of a drug problem, although some are commonly used or have a relatively strong empirical foundation, such as the definition proposed in the Diagnostic Statistical Manual, fourth edition (DSM-IV). The DSM-IV distinguishes between two types of drug problems: substance abuse disorder and substance dependency disorder, with the latter usually termed addiction. Other alternative means of defining a drug problem are the Addiction Severity Index (ASI) and other instruments modeled on the ASI. In discussing key elements in defining a "drug problem," this paper discusses validating an assessment instrument against a specific definition, the reliability and validity of assessment instruments, and criteria for evaluating and comparing assessment instruments. Remaining sections of the paper discuss recommended drug screening and assessment instruments, screening and assessment practices in juvenile justice, and screening and assessment implementation challenges. The paper concludes that researchers currently lack information on the full range of instruments in use among juvenile justice systems and stages within these systems. The available evidence indicates that many instruments are being used and that few have been rigorously validated. Meanwhile, practitioners have been given relatively little guidance on which instruments to use among the many available. 2 tables and 67 references