M.tb is an intracellular organism and protective immunity is dependent on an intact cellular immune system. Both Class II-restricted CD4+ and Class I-restricted CD8+ T lymphocytes are important for protection. Gamma delta cells and non-classically restricted T cells such as CD1-restricted T cells may also play a protective role but knowledge of how to induce these to protective levels by vaccination is very limited. Although several vaccine delivery systems are capable of inducing T cells, e.g. protein/adjuvant combinations, DNA vaccines and recombinant viral vectors, when used alone these delivery systems induce only low-level responses. Heterologous prime-boost immunisation strategies involve immunizing with 2 two different vaccines, each expressing the same antigen, several weeks apart. These strategies induce higher levels of CD4+ and CD8+ T cells than homologous boosting in animal models of malaria, HIV and TB.