Platelet-rich plasma (PRP) and stem cell (SC) injections have become increasingly common in the treatment of knee arthritis. This systematic review was performed to answer the following questions: (1) What effects does intraarticular PRP injection have in the setting of knee arthritis? (2) What effects does intra-articular SC injection have in the setting of knee arthritis? (3) What adverse events have been reported in the literature from PRP injections for knee arthritis? (4) What adverse events have been reported in the literature from SC injections for knee arthritis? [Orthopedics. 2021;44(6):376-383.].