Abstract: Whether called addiction, abuse, or dependence, patients with substance use disorders generally show a combination of physical indicators, a variety of serious or ongoing negative consequences of drug use that affect significant areas of their lives, and an apparent compulsion to seek and use drugs despite ongoing negative consequences. This chapter focuses on the assessment of substance use disorders in adults. It begins with a review of the nature of the disorders, which is followed by a review of clinical assessment instruments designed for the assessment purposes of (a) diagnosis, (b) case conceptualization and treatment planning, and (c) treatment monitoring and evaluation. Recommendations are included for instruments with the greatest scientific support and for assessing substance use disorders in a clinically useful manner.