Eating disorders and alcohol/drug abuse are frequently comorbid. Eating-disordered patients are already at an increased risk for morbidity and mortality, so alcohol and drug use pose additional dangers for these patients. Restricting anorexics, binge eaters, and bulimics appear to be distinct subgroups within the eating-disordered population, with binge eaters and bulimics more prone to alcohol and drug use. Personality traits such as impulsivity have been linked to both bulimia nervosa and substance abuse. Many researchers have proposed that an addictive personality is an underlying trait that predisposes individuals to both eating disorders and alcohol abuse. Interviewing is generally the most useful tool in diagnosing alcohol and substance abuse disorders in individuals with eating disorders. It is essential for the physician to be non-judgmental when assessing for substance abuse disorders in this population. We discuss interviewing techniques, screening instruments, physical examination, and biological tests that can be used in evaluating patients with comorbid eating disorders and substance abuse. More studies are needed to understand psychobiological mechanisms of this comorbidity, and to develop treatments for individuals with comorbid eating disorders and substance misuse.