Various kinds of thermometers have been developed for practical uses in clinical and physiological measurements. Most of them, however, do not fully satisfy all the requirements in body temperature measurement. An electronic thermometer makes use of a thermistor or PN junction sensor to measure body temperature. In addition, a crystal and thermocouples are applied for the thermometer. On the other hand, several kinds of infrared detectors, such as thermal, photoconductive or photovoltaic detectors, have been developed and applied in radiation thermometer and thermograph system. As a noninvasive measurement method, a zero heat flow thermometer has been developed for measuring temperature in the central parts of animal body. In monitoring animal body temperature continuously a conventional measurement system is not suitable for such regions as rectum, vagina and tympanum for a longterm use. However, a zero heat flow thermometer can be used regardless of sex or region for animal body temperature measurement. The radiation thermometer in animal use has been considered accurate enough for use as a medical thermometer. The measuring region of skin temperature is limited only to the body without hair covering its skin surface. This paper presents techniques for measuring body temperature and discusses the electronic thermometer for the use in animals.