The World Health Organization defines latent tuberculosis infection (LTBI) as a state of persistent immune response to stimulation by the antigens of Mycobacterium tuberculosis, without evidence of clinically manifested active disease. LTBI has clinical implications. Positive tests for LTBI may give the false impression of active tuberculosis. As well, the latent state has a potential for reactivation in the future. This chapter explores the medical conditions that increase the risk for tuberculosis reactivation and reviews the diagnostic utility, intent and relevance for testing for latent tuberculosis. It discusses the technique of Tuberculin skin testing and the conditions in which the Tuberculin test may be falsely-positive or negative, respectively. It also compares the relevance of the Tuberculin skin test with the Interferon-gamma based test.