Ten percent of the population abuses drugs or alcohol, and 20 percent of patients seen by family physicians have substance-abuse problems, excluding tobacco use. These patients can be identified by relying on regular screening or a high index of suspicion based on "red flags" that can be noted in various clinical situations. The modified CAGE questionnaire is an excellent screening instrument, but several alternatives are available. The best screening test is one that the physician will routinely use well. Laboratory indicators such as gamma-glutamyl transpeptidase, mean corpuscular volume, and carbohydrate-deficient transferrin are nonspecific but can add to the evidence of alcohol abuse. If problem alcohol use is diagnosed, even brief physician advice can be helpful. If the problem has progressed to addiction, referral to an addiction specialist or treatment center is recommended. Special issues arise in dealing with substance abuse in adolescents, elderly patients, and patients with mental illness, but the family physician can play an important role in recognizing this common problem.