Among infectious diseases, tuberculosis is the leading cause of death, killing around 3 million people each year. Most cases occur in young adults but it is also a major cause of illness and death in children. The problem has been exacerbated in recent years by the HIV pandemic and by the emergence of multidrug resistance. Co-infection with HIV greatly enhances the risk of overt tuberculosis and in 1999 it is expected that tuberculosis will account for 30% of the predicted 2.5 million AIDS-related deaths. By inducing clinically and radiologically atypical forms of tuberculosis, and by increasing pressure on diagnostic facilities by sheer numbers, serious diagnostic difficulties are increasingly occurring in both adults and children in the tropics. At the present time, 2% of all cases of tuberculosis are multidrug resistant but, as the treatment of such cases is often grossly inadequate in many tropical countries, their frequency will doubtless grow. There are no simple solutions to the global emergency of tuberculosis: clearly there is a need for better use of available control measures but there is also a need to reach a much clearer understanding of the underlying immune phenomena in this disease so as to develop more effective vaccines and therapeutic agents. Finally, it cannot be ignored that tuberculosis is a disease of poverty--95% of cases and 98% of deaths due to it occur in the developing nations--and thus a major control measure is a resolution of the gross inequities in health care provision both between and within nations.