Recombinant DNA techniques have enabled the selection and production of any specific gene and thus allowed the development of nucleic acid hybridization techniques as diagnostic tests for microbial identification. In the hybridization reaction sample nucleic acids, rendered single stranded by denaturation, are allowed to anneal with known specific probes carrying isotopic or nonisotopic label thus enabling the homologous target sequences in samples to be identified. Several hybridization tests have been established and used for the detection of many subviral, viral, bacterial and eukaryotic pathogens, which are difficult to diagnose by other methods. The most important challenge now for the hybridization techniques is in the development of practical tests, simply and rapidly performed, with high sensitivity, automation and nonisotopic detection system.