In an attempt to explain the stresses encountered in the plastics industry, this article first defines the different types of internal stresses in amorphous polymers. Each type of thermal stress is then discussed in detail, with reference to the mechanism of generation and the effect on engineering properties. Methods of detecting and measuring internal stresses are also presented. The article then describes the combined effects of thermal stresses and orientation that result from processing conditions. Finally, it discusses numerous aspects of physical aging and the use of high-modulus graphite fibers in amorphous polymers.