The incidence of tuberculosis is on the rise, with a reported 18% increase between 1985 and 1992. There is every reason to think that this trend will continue in the 1990s. Expectedly, there is a renewed interest in early detection and prevention of this potentially life threatening disease. Tuberculin skin testing remains a valuable diagnostic tool for the detection, control, and elimination of tuberculosis. It involves the intracutaneous injection of culture extracts of tubercle bacilli as a classic example of delayed hypersensitivity reaction. Despite proper technique and interpretation of the test, its usefulness is limited by cross-reaction caused by other mycobacterial antigens. False-negative reaction is a common confounding problem. It is therefore important to interpret the test based on the overall clinical context of the individual being tested. This article aims to review the basic immunology, proper technique, interpretation, and indication for the test.