Because of the prevalence of substance abuse in general clinical populations, it is important for psychologists to have knowledge and skill in this area. Psychologists also have special expertise to offer in the assessment and treatment of alcohol/drug problems. Current evidence indicates that (a) alcohol/drug problems generally obey ordinary behavioral principles and processes, (b) substance abuse frequently occurs within a broader cluster of psychological problems, (c) the treatment approaches most strongly supported by outcome research are fundamentally psychological in nature, (d) cognitive-behavioral principles are of demonstrable value in motivating change in alcohol/drug use, and (e) clinical skills and styles (e.g., empathy) commonly included in the training of psychologists are important determinants of favorable treatment outcomes with substance use disorders. These factors in the context of changing health care indicate that psychologists should play an increasing role in assessing and treating addictive behaviors.