The past decade has witnessed an explosion in the knowledge of microbial genetics, pathogenesis, and antimicrobial resistance as a result of advances in molecular technology. This has brought important breakthroughs in the management of patients with infectious diseases, as organisms that had previously been difficult to demonstrate in vitro can now be detected by molecular techniques such as the polymerase chain reaction. Not only is rapid diagnosis now possible, but old diseases of uncertain aetiology have been found to have an infective origin, for instance, Whipple's disease. Molecular technology has also contributed greatly to epidemiological studies of outbreaks, understanding antimicrobial resistance, developing new antimicrobial agents, the in vitro synthesis of immunomodulators, production of vaccines, and gene therapy. The limitations of these latest technologies, however, need to be remembered so that they yield meaningful information for patient care.