Tuberculosis (TB) contacts are people who have close contact with patients with infectious TB. As they are at high risk for infection (and in line with the Stop TB strategy), TB contacts should be investigated systematically and actively for TB infection and disease. Such interventions are called ‘tuberculosis contact investigations’. They contribute to early identification of active TB, thus decreasing its severity and reducing transmission of Mycobacterium tuberculosis to others, and identification of latent TB infection (LTBI), to allow preventive measures. Contacts are commonly investigated in high-income countries with low TB burdens and in settings in which a TB elimination policy is implemented, in order to identify persons with early active TB or who have recently been infected. People identified as infected are then treated for LTBI with isoniazid for at least 6 months (usually 9 months) or with shorter combination regimens including isoniazid and rifampicin. TB contact investigations are rarely and inconsistently carried out in resource-limited settings. In most low- and middle-income countries, it is included in the national policy to control and prevent TB; however, in the vast majority of countries, it is either not undertaken or is implemented on the basis of no or poor standards, because of the absence of clear definitions of index cases, contacts and procedures. Furthermore, the health personnel who should be involved are usually not clearly identified. Information on the contribution of routine contact investigations to early TB case detection is scarce in these countries or is non-standardized, thus precluding an assessment of its impact on reducing transmission. Many studies in countries with a high TB incidence have shown that the prevalence may reach 5% or more among contacts, particularly among household members. Other data suggest that contact investigations could be particularly useful for identifying childhood TB. Furthermore, contact investigation can help identify people who require careful follow-up, such as those who were exposed to an index case of multi-drug-resistant or extensively drug-resistant TB or people infected with HIV, whose risk for rapid progression to active TB is very high.