Tuberculosis is a major public health problem in many regions of the world, accounting for up to 2 million deaths per year. The main causative agent of tuberculosis is Mycobacterium tuberculosis, which usually infects the lungs, causing symptoms such as fever, cough and chest pain. After infection, the bacilli can be cleared by a healthy immune system or, on the other hand, the infection may lead to latent tuberculosis or progress to active pulmonary tuberculosis. Moreover, in certain situations, such as cases of immunocompromised patients, these bacilli can spread through the lymphatic or hematogenous route to more distant organs, leading to extrapulmonary tuberculosis. Approximately 30% of the population worldwide has latent tuberculosis and 5 to 10% of individuals infected with M. tuberculosis progress to active tuberculosis disease. Although tuberculosis is a curable disease, some factors have been associated with the morbidity and mortality of global tuberculosis, including inadequate detection, increased abandonment rate of anti-TB treatment, co-infection with human immunodeficiency virus, increased drug resistance and inefficiency of the BCG vaccine in preventing the disease in adolescents and adults. New tools are therefore urgently needed to control tuberculosis. Knowledge of the molecular characteristics, immune response and pathogenesis of M. tuberculosis are essential for the development of new approaches to diagnosis, drugs and vaccines to assist in the clinical management and control of the disease. This first chapter on tuberculosis describes the epidemiological, immunological and clinical aspects of the disease, as well as the microbiologic and molecular characteristics of M. tuberculosis.