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Aminoacyl-tRNA synthetases catalyse a key reaction in protein biosynthesis. They match the 20 amino acids to the genetic code by specifically attaching them to their adaptors, transfer ribonucleic acid (tRNA) molecules. The reaction proceeds in two steps: the amino acid is first activated by adenosine triphosphate (ATP) to form aminoacyl adenylate, and then the aminoacyl group is transferred to the terminal ribose of tRNA. This family of enzymes is divided into two classes, based on the similarities in primary structure and architecture of the active site domains; the two architectures are characterized by two modes of binding of ATP, the intermediate aminoacyl adenylate and the acceptor end of tRNA, which result in two regioselectivities of amino acid attachment to the terminal ribose of the tRNA. Aminoacyl-tRNA synthetases are modular enzymes; to the central active site module are attached various domains with diverse functions such as tRNA-binding and amino acid editing. The primary subject of this article are structural and functional aspects of these enzymes. Key Concepts Aminoacylation is a key reaction in protein biosynthesis, as it matches the standard 20 amino acids to the genetic code. Aminoacylation is generally catalysed by aminoacyl-tRNA synthetases, a family of 20 enzymes, one for each standard amino acid and a set of corresponding (cognate isoacceptor) tRNAs. Aminoacylation proceeds in two steps, where the amino acid is first activated by ATP to form aminoacyl adenylate, a mixed anhydride, and then transferred to form an ester bond with the terminal ribose of tRNA. Aminoacyl-tRNA synthetases are divided into two classes, based on the similarities in primary structure (sequence) and architecture of their active site domains. Aminoacyl-tRNA synthetases are modular enzymes; to the central active site module are attached various domains with diverse functions such as tRNA-binding and amino acid editing. Aminoacyl-tRNA synthetases are highly specific enzymes, attaching the correct amino acids to the corresponding tRNAs with high fidelity. Aminoacyl-tRNA synthetases of the two classes generally bind the acceptor arm of tRNA in two ways that constitute mirror images of each other, resulting in two different regioselectivities of amino acid attachment to the tRNA. Keywords: structure and function; aminoacylation; translation; protein synthesis; transfer ribonucleic acid (tRNA)