A review is presented of the activity of our research group in the area of mid-infrared (MIR) fiber-optical chemical sensing systems based on the new generation of infrared-transparent fiber materials such as chalcogenide (As-Se-Te) and silver halide (AgBr-AgCl). The availability of these fiber materials extends the optical window utilizable for chemical-sensor systems significantly into the MIR range (2-20 μm). The increasing number of publications on this topic during recent years attests to the fascinating development of this new method. The aim of this review is to establish the state-of-the-art capabilities of chemically modified IR-transparent fibers as chemical IR sensors for the on-line analysis of IR-active compounds in aqueous solutions (chlorinated hydrocarbons, glucose) and in the gas phase (chlorofluorohydrocarbons) as developed in our laboratory.