Injection molding is the most widely used processing technique for polymers. It offers several advantages over other processing conditions such as good surface finish, the ability to process complex parts without the need of secondary operations, and low cost for mass production. However, because of the complex deformation, and thermal and pressure histories that the polymer melt experiences during processing, residual stresses develop. These stresses act internally at room temperature and have the same effects on the material as externally applied stresses do, resulting in shrinkage and warpage of the product. In recent years, with the development and use of engineering plastics in an increasing number of applications, and with the tougher quality control policies in industries such as the automotive, the effects of residual stresses in product quality and performance have raised great interest. This review reports up-to-date advances in the field of residual stresses developments in polymers, with special attention given to injection molded products. Flow- and thermal-induced residual stresses are reported. Emphasis is given to the processing parameters that most influence residual stresses during injection molding as well as the effect of residual stresses not only on warpage but also on other material properties.