The construction and theory of operation of thermistor detectors is reviewed. The effects of changes in ambient temperature are discussed and measures for accommodating or compensating for these effects are presented. System conditions where thermistors would be most applicable are described and a comparison made with other thermal detectors. Equations for calculating responsivity and D* are given and the improvement resulting from immersion. Unimmersed thermistor detectors will generally have D*s in the range 1 - 3 (10)8 depending on the time constant. Immersion can increase this by a factor of 3.5.