Tuberculosis is one of the major health threats worldwide; 2 million people die each year from this disease and about a third of the world's population is infected with Mycobacterium tuberculosis. Existing drugs against this bacterium are several decades old and with the alarming increase in multidrug-resistant and extensively drug-resistant strains new drugs are urgently needed. Currently the ultimate success of anti-tuberculosis therapy is the determination of the relapse rate within the first two years aft er initially successful treatment. However, because of the long duration of clinical trials that rely on this outcome the pharmaceutical industry is reluctant to develop new anti-tuberculosis drugs. Therefore, biomarkers of treatment response and outcome are urgently needed. Certain host markers such as cytokines that are expressed and secreted in response to mycobacteria by macrophages and T cells could possibly be useful biomarkers for tuberculosis treatment response and prediction of treatment outcome. This chapter describes the host immune response to Mycobacterium tuberculosis and in particular the cytokine expression and secretion in humans latently infected with this bacterium as well as in individuals with active tuberculosis disease. Furthermore, the changes in cytokine release during tuberculosis treatment are discussed. Additionally, the usefulness of these cytokines as biomarkers to differentiate between latent infection and active disease and as biomarkers for tuberculosis treatment response and outcome is highlighted.