Usually, the temperature in boreholes is determined using a standard temperature probe. The logging technique is either "stop and go", or the probe is lowered as a moving probe into the borehole using a controlled speed. Distributed temperature probe arrays installed permanently in a borehole are an alternative to moving probes and can be applied especially for temperature monitoring even under conditions where moving probes cannot be used. The distributed optical fiber sensing technique represents a new approach for temperature measurements. The basis for this method is given in Boiarski (1993), Dakin et al. (1985), Farries and Rogers (1984), Hartog and Gamble (1991), Rogers (1988), Rogers (1993). First results using fiber optic temperature sensing in boreholes and temperature monitoring for studying geotechnical and environmental problems (e.g., waste deposits) are published in Hurtig et al. (1993; 1994; 1995) and Hurtig and Schroetter (1993).