In recent years new information has been collected about the immunological responses to pathogenic mycobacteria. More information on cellular and molecular responses of cells in murine and human tuberculosis has been produced and T cells' role in the production of selected cytokines has been clarified. Studies in mice have provided insight into the phases of the T cell response to virulent M. tuberculosis, the role of various T cell subsets, and the repertoire of antigens recognized by these cells. However, despite this new information, some of which has been confirmed in humans, large gaps remain in our knowledge about the immune response to this infection, particularly concerning cellular or molecular mechanisms involved in acquired protection. Even if some extrapolations from adult data can be made, large gaps in our knowledge exist on the potential immune defects in young infants who are prone to develop tuberculosis soon after infection.