**Abstract**

This chapter presents a brief description of conductive polymer composites in general, with more attention paid to those exhibiting abrupt change of resistivity when temperature raises, associated with PTC and NTC (respectively, positive and negative temperature coefficient of resistivity) effects. These materials are "smart" because they can adapt their electrical behavior to environmental characteristics, hence being suitable for temperature sensors, smart heating devices, safe batteries, and resettable fuses. As compared to NTC, the applications of PTC materials are more numerous, because the abrupt increase of resistivity with temperature rise allows the current and temperature to be naturally limited. The PTC effect and the factors controlling its quality, e.g., repeatability, intensity, switching temperature, and subsequent NTC effect, are discussed from the point of view of the influence of the nature of polymeric matrix, conductive fillers, and applied treatments. Increased attention is paid to composites with conductive carbonaceous fillers, and these materials being of great interest because they have considerably lower density than metals, are easier to process, and can impart surprising mechanical and electrical properties to polymer matrices. Examples and applications of temperature sensors based on PTC composite materials, applications, and perspective aspects are discussed within the chapter.

**Keywords:** temperature sensors, thermistor, positive temperature coefficient, conductive polymer composites, carbonaceous materials
