Recycled plastics show in many cases inferior properties during manufacturing and in use compared to virgin alternatives. To upgrade recyclates to the requested virgin-like performance reformulation through addition of suitable additives is mandatory taking the structural differences between virgin and recycled plastics into consideration. Plastics are degraded during all processing steps and use in general through oxidation processes. Therefore, restabilization of recyclates through antioxidants and for outside applications through UV/light stabilizers is essential to improve processing behavior and long-term properties, whereas recyclate-specific additive systems outperform traditional stabilizers. Degradation of polyesters or polyamides through hydrolysis can be repaired through chain extenders and the pristine mechanical properties regained. Compatibilizers enable the high-value use of incompatible polymer blends or compensate the negative influence of impurities. Odor issues are addressed by adding reactive molecules or adsorbents. The presented additive concepts of the chapter summarize the various possibilities to upgrade recycled plastics and to provide recycled materials of high quality.