The natural history and clinical expression of infection due to Mycobacterium tuberculosis differ substantially in children compared with adults. The natural history depends upon the age at infection and the host immune status. Children infected prior to age 4 have a very high rate of developing immediate clinical or radiographic manifestations or both, but are unlikely to develop reactivation disease in adulthood. In contrast, children infected in preadolescence or adolescence are more prone to developing more severe adult-type pulmonary tuberculosis soon after infection or in adulthood. It is difficult to confirm the diagnosis of tuberculosis by current microbiological methods. Even in industrialized countries, the triad of a positive tuberculin skin test, radiographic and/or clinical manifestations consistent with tuberculosis, and establishing a recent link to a known infectious case of tuberculosis is the "gold standard" for diagnosis. Children with tuberculosis respond well to and tolerate the same basic treatment regimens as used for adults. Some prevention of childhood tuberculosis can be achieved by the use of the bacille Calmette-Guérin (BCG) vaccines, but the use of chemotherapy to treat recent tuberculosis infection, discovered via contact tracing, is of paramount importance even when BCG vaccines are used.