The investigation of the development and properties of biofilms is difficult because classical microbiol. offers only destructive methods apart from microscopical observation. The paper presents the FTIR spectroscopy as a means to investigate microorganisms in biofilms. Furthermore, in addn. to taxonomic and genetic methods for the identification of microorganisms, the FTIR anal. provides fingerprint spectra, allowing the rapid characterization of microbial strains. The FTIR-ATR (reduced total reflection) technique can be used for the observation of biofilms forming directly on the ATR crystal. Spectra can be gained non-destructively, in situ, and in real time. The method is suitable for fundamental biofilm research as well as for monitoring of biofilm formation, e.g., in an ultrapure water system. It also allows the rapid anal. of deposits on filtration membranes or other surfaces and supports the discrimination between microorganisms, inorg. material, or other foulants. The potential of the diffuse reflexion (DRIFT) is emphasized. With the DRIFT method it is possible to investigate rough surfaces or pulverized material and to detect biomass or other surface contaminants. The examples demonstrate that the FTIR spectroscopy holds a powerful potential for biofilm anal. and can be applied in manyfold ways.