This chapter discusses thermal conductivity of polymers. The chapter tabulates the values of thermal conductivities and temperature for pure polymers, commercial foamed polymers, polymers reinforced with inorganic fillers, and thermally conductive polymer composites. The effect of temperature on polymers is of practical importance because most polymers are processed at relatively high temperature and have applications in a wide temperature range. Generally, pure polymers have low thermal conductivities, ranging from 0.1 to 0.6 W/(m K). Foaming polymers may further enhance this low thermal conductivity. The thermal properties of the filler, the size, shape, and orientation of filler particles or fibers in polymer matrix, and the percentage of fillers are all important factors that determine the thermal conductivity of reinforced polymers. Polymers reinforced with inorganic fillers usually increase their thermal conductivities from a few percent to a few times. Highly thermal conductive polymer composites can be obtained by using fillers with high thermal conductivity and at high percentages.