the development of a simple but very effective modular fiber-optic fluorimeter is described. using this instrumentation, different fiber-optic chemo- and biosensors are investigated to demonstrate the application of optical sensors in biotechnology. to obtain information about vital biomass concentrations and the metabolic state of organisms during cultivation, a non-invasive fast-responding on-line sensor based on the combination of turbidometric and spectrofluorometric methods (culture fluorescence monitoring) is presented. in another system, permeabilized cells are confined in front of the fiber tip and changes in the nad(p)h pool of membrane-bound enzyme complexes inside the cells are monitored during enzymatic reactions with different analytes to study the metabolic activity of the system and to use the system as a biosensor. another fiber-optic sensor uses the effect of luminescence quenching of a special ru(ii) complex by oxygen to form a sterilizable oxygen optode for measuring dissolved and exhaust-gas oxygen levels. the development of a glucose sensor is also illustrated