Thermistors are components of comparatively recent development, although the phenomenon of conduction in metallic salts was recorded by Faraday over a century ago.The paper, in a broad survey of the subject, describes the theory of their operation, explains why they, in common with other semiconductor devices, possess a negative temperature coefficient of resistance and develops the expressions which govern their parameters.Methods of manufacture of the various forms of thermistor are outlined in general terms, and numerous applications of the device are described and discussed. Guidance is given on the approach to circuit problems, and the possibilities of exploiting some of the device's interesting and unusual properties are indicated. The point is made that the thermistor can now be used in many fields as an adequate alternative to components of more specialized application.