This handbook is intended for use as a reference guide by staff members of drug abuse treatment programs by providing information on available diagnostic screening and assessment tools for use with adolescent and adult substance abusers. The handbook discusses the benefits of using standardized assessment tools, how to select the tools, and how to conduct a comprehensive screening. The criteria to consider when selecting an instrument include: whether the instrument yields useful information for evaluating and diagnosing the client's problems; whether the information yielded is quantifiable; whether the instrument is easy to administer and is not excessive in length; whether the wording and language of the instrument are appropriate for the targeted client; whether the instrument is sufficiently sensitive to the relevant problems of the client; and the cost of materials required for using the instrument. The information in this handbook is presented in two main sections: assessment instruments for adolescent drug users and assessment instruments for adult drug users. The section on adolescent instruments examines 6 comprehensive screening assessment instruments and 29 instruments that are useful for brief screening, evaluation of particular problem areas, and establishment of a DSM-III diagnosis. The section on adult instruments examines 3 comprehensive screening assessment instruments and 27 additional instruments that are useful for brief screening, evaluation of particular problem areas, and establishment of DSM-III or DSM-III-R diagnosis. Tables and references