Recent developments in medical technology have caused a great change in infectious diseases, as characterized by epidemics of antibiotics-resistant bacteria, opportunistic infection in compromised hosts, and blood-borne viral infections such as hepatitis virus and human immunodeficiency virus. In the diagnosis of such new aspects of infectious diseases, conventional immunological, culture, and microscopical techniques are not always practical. By contrast, evaluation of infectious agents using molecular biological technology frequently offers the rapid, most accurate and sensitive method of diagnosis. Amplification methods are particularly attractive for the detection of small numbers of microorganisms, as in latent conditions, or for the fastest identification of the pathogen without laborious isolation. For introduction of the tests into routine procedures, their standardization as well as simplicity and low cost are required. Gene level diagnostics should be applied appropriately to management of infectious diseases along with the conventional techniques, while further roles of the tests must be determined, on the basis of the molecular elucidation of infectious diseases.