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The immunologic mechanisms of latent tuberculosis (TB) infection are complex and hitherto not completely understood. The lifelong risk of an immunocompetent individual of developing active TB after infection with M. tuberculosis is 5-10 % and highest during the first two years after infection. Various factors may considerably increase the risk of developing active TB, e. g., immunosuppressive disease or immunosuppressive medication. However, the development of active TB may be avoided by preventive chemotherapy, the therapy of choice being isoniazid over a 9-month period. Alternative treatment regimens may be indicated in special cases, but it must be borne in mind that the efficacy of these regimens has not been studied sufficiently while they seem to be less well tolerated than isoniazid monotherapy. The tuberculin skin test is still the only sufficiently documented method to detect latent infection with M. tuberculosis which is also suitable for routine application. This test today should be performed exclusively as described by Mendel and Mantoux. Its sensitivity and specificity depend on the prevalence of tuberculosis infection. It should therefore be restricted to individuals at increased risk of latent TB infection. When interpreting the tuberculin skin test, it is necessary to know whether an individual belongs to one of the defined risk groups or has an elevated risk of developing active TB. Among the risk groups are individuals who may have been infected recently with M. tuberculosis (contacts of contagious TB patients) or in whom other factors increase their risk of developing active TB. The indication for chemotherapy for latent TB infection must be based on a careful individual risk-benefit analysis and, besides patient compliance, requires full information of the patient and careful monitoring during therapy. Before initiating treatment, active TB must always be excluded by the proven methods.