At first the principle of nucleic acid hybridisation, some important technics used heretofore, and methods to label nucleic acids are described. The advantages and disadvantages regarding to the application to microbiological diagnostics are discussed. The advantage, before all, is the high specificity of the test which allows to detect the presence and the properties of genes which are not expressed. The methods known up to now can only be applied if the probe is labelled radioactively, since under these conditions the sensitivity is high enough to identify bacteria contained in clinical isolates without prior cultivation. The comparable complex methods are restricted, presently, to special mostly epidemiological problems. To improve these technics regarding increased sensitivity, to the use of non-radioactively labelled probes, to higher speed, and to the automation of the test internationally much work is carried out with great intensity. The solution of these problems will create conditions for a wide application of DNA probes in the general microbiological laboratory.