Three phys. methods for monitoring of biofouling are presented: a fiber optical device; a differential turbidity measurement device; and an FTTR flow cell. The first two methods are based on light reflectance and detect the deposition of reflecting material. Thus, they are not specific for biofilms but they allow us to detect deposit formation in situ, non-destructively and in real time. The third method gives information about the chem. nature of the deposit, allowing us to identify biol. material. However, this increase of information requires a significantly higher tech. effort.