A review with 32 refs. The investigation of the development and properties of biofilms is difficult because classical microbiol. does not offer non-destructive methods other than microscopical observation. This paper present different techniques of FTIR spectroscopy as a means to investigate microorganisms in biofilms. Furthermore, in conjunction with taxonomical and genetical methods it is now suitable for the identification of microorganisms. The FTIR anal. provides fingerprint spectra, allowing the rapid characterization of microbial strains. The FTIR-ATR technique can be used for the observation of biofilms forming directly on the surface of an ATR crystal. Spectra can be obtained 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. FTIR-ATR also allows the rapid anal. of deposits on filtration membranes or other surfaces and supports the distinction among microorganisms, inorg. material and other foulants. With the DRIFT-technique it is possible to investigate rough surfaces or very small samples and to characterize biomass or other surface contaminants. The examples demonstrate that FTIR spectroscopy holds a considerable potential for biofilm anal. and physiol. studies and can be applied in many ways.