Monitoring of substrates and products during fermentation processes can be achieved either by on-line, in situ sensors or by semi-on-line analysis consisting of an automatic sampling step followed by an ex situ analysis of the retrieved sample. The potential risk of introducing time delays and signal bias during sampling makes it necessary to distinguish between real-time, on-line, in situ methods and semi-on-line analysis. In addition, semi-on-line analyzers are often mechanically complex-a circumstance which has to be given special attention during their industrial use on a routine basis. This review on semi-on-line analysis will focus both on the dynamics and precision of aseptic sampling devices and on the performance of flow injection analysis (FIA) and sequential injection analysis (SIA), especially with regard to their robustness when used in industry.