Exonucleolytic proofreading of DNA synthesis errors is one of the major determinants ofgenome stability. However, many DNA transactions that contribute to genome stability requiresynthesis by polymerases that naturally lack intrinsic 3´ exonuclease activity and some of whichare highly inaccurate. Here we discuss evidence that errors made by these polymerases may beedited by a separate 3´ exonuclease, and we consider how such extrinsic proofreading may differfrom proofreading by exonucleases that are intrinsic to replicative DNA polymerases.