The canonical functions of aminoacyl-tRNA synthetases (aaRS) are charging amino acids to their cognate tRNAs to ensure the precise protein synthesis. Over the course of evolution, aaRS progressively incorporated domains and motifs that have no essential connections to tRNA charging but play roles in cell signaling. These include mediating protein-protein interaction, protein subcellular localization and sensing and transmitting metabolites signals. Deregulated noncanonical aaRS functions are associated with array of human diseases. These all suggest that aaRS play roles beyond their tRNA charging activity, however, the underlying biochemical mechanisms remain to be elucidated. Recent studies revealed that aaRS have aminoacyl transferase activities. An amino acid can be specifically recognized and activated into aminoacyl-AMP by its cognate aaRS, and the formed aminoacyl-AMP in aaRS can modify lysines in proteins that physically interact with this aaRS. This aminoacylation senses and transmits amino acids abundance and side chain information into cell signaling network, provides opportunities to understand why additional domains are acquired by aaRS during evolution and how mutations in an aaRS causes specific human diseases. This review summarizes the noncanonical functions of aaRS and discusses how aaRS mutations may be linked to diseases.