During the last decade, there has been a marked increase in the number of gravity of tuberculosis cases both in developing countries and in industrialized nations. This is, in part, due to the acquired immune deficiency syndrome (AIDS) pandemic, but global economic depression, increased homelessness and declining control programmes have also contributed. One of the more insidious consequences of this resurgence has been the recent emergence and nosocomial transmission of multidrug-resistant strains of Mycobacterium tuberculosis, thus raising the possibility that untreatable forms of the disease may become widespread. Somewhat surprisingly, given the difficulties of working with this slow-growing pathogen, remarkable progress has been made in a relatively short time, in understanding the molecular epidemiology, the genetic basis, and the biochemical mechanisms of drug resistance. Furthermore, a number of promising molecular tools are now available to help counter tuberculosis and to further understanding.