In many liquid crystal systems there is a direct phase transition between an optically scattering smectic or chiral nematic mesophase and the clear isotropic phase. Such transitions are characteristically sharp and take place at well defined temperatures which may occur anywhere in the range -20°C to 200°C. It is this novel feature of liquid crystal systems which together with the attributes of low loss optical fibres forms the basis of an accurate optical temperature switch'. The properties of this device include remote passive sensing with a narrow switching differential which is safe in hazardous environments, and is free from EM and RF interference. In this paper we discuss the operation of the liquid crystal temperature switch in a number of device configurations (reflective or transmissive) together with the wide variety of applications.