In thermosets and semicrystalline and/or amorphous thermoplastics, processing can result in internal molecular stress, which relieves on reheating (thermal history effects). The relief of these stresses appears as a small endothermic relaxation even after glass transition. Non reversing transitions in polymers also indicate the existence of thermal history effects. These effects can be modeled thereby improving interpretation of many experimental results. In this paper, a mathematical model for thermal history effects and heat flow in thermoplastics has been developed.