Translating the 4-letter code of RNA into the 22-letter alphabet of proteins is a central feature of cellular life. The fidelity with which mRNA is translated during protein synthesis is determined by two factors: the availability of aminoacyl-tRNAs composed of cognate amino acid:tRNA pairs and the accurate selection of aminoacyl-tRNAs on the ribosome. The role of aminoacyl-tRNA synthetases in translation is to define the genetic code by accurately pairing cognate tRNAs with their corresponding amino acids. Synthetases achieve the amino acid substrate specificity necessary to keep errors in translation to an acceptable level in two ways: preferential binding of the cognate amino acid and selective editing of near-cognate amino acids. Editing significantly decreases the frequency of errors and is important for translational quality control, and many details of the various editing mechanisms and their effect on different cellular systems are now starting to emerge.